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Evolution of HTTP - HTTP | MDN
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HTTP over QUIC</a></li><li class="document-toc-item "><a class="document-toc-link" href="#see_also">See also</a></li></ul></section></div></div><div class="sidebar-body"><ol><li class="section"><a href="/en-US/docs/Web/HTTP">HTTP</a></li><li class="section">Guides</li><li><a href="/en-US/docs/Web/HTTP/Overview">An overview of HTTP</a></li><li><a href="/en-US/docs/Web/HTTP/Session">A typical HTTP session</a></li><li><a href="/en-US/docs/Web/HTTP/Messages">HTTP messages</a></li><li><a href="/en-US/docs/Web/HTTP/MIME_types">MIME types (IANA media types)</a></li><li><a href="/en-US/docs/Web/HTTP/Compression">Compression in HTTP</a></li><li><a href="/en-US/docs/Web/HTTP/Caching">HTTP caching</a></li><li><a href="/en-US/docs/Web/HTTP/Authentication">HTTP authentication</a></li><li><a href="/en-US/docs/Web/HTTP/Cookies">Using HTTP cookies</a></li><li><a href="/en-US/docs/Web/HTTP/Redirections">Redirections in HTTP</a></li><li><a href="/en-US/docs/Web/HTTP/Conditional_requests">HTTP conditional requests</a></li><li><a href="/en-US/docs/Web/HTTP/Range_requests">HTTP range requests</a></li><li><a href="/en-US/docs/Web/HTTP/Content_negotiation">Content negotiation</a></li><li><a href="/en-US/docs/Web/HTTP/Connection_management_in_HTTP_1.x">Connection management in HTTP/1.x</a></li><li><em><a href="/en-US/docs/Web/HTTP/Evolution_of_HTTP" aria-current="page">Evolution of HTTP</a></em></li><li><a href="/en-US/docs/Web/HTTP/Protocol_upgrade_mechanism">Protocol upgrade mechanism</a></li><li><a href="/en-US/docs/Web/HTTP/Proxy_servers_and_tunneling">Proxy servers and tunneling</a></li><li><a href="/en-US/docs/Web/HTTP/Client_hints">HTTP Client hints</a></li><li class="toggle"><details><summary>Security and privacy</summary><ol><li><a href="/en-US/docs/Web/Security/Practical_implementation_guides">Practical security implementation guides</a></li><li><a href="/en-US/observatory">HTTP Observatory</a></li><li><a href="/en-US/docs/Web/HTTP/Permissions_Policy">Permissions Policy</a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/CSP">Content Security Policy (CSP)</a></li><li><a href="/en-US/docs/Web/HTTP/CORS">Cross-Origin Resource Sharing (CORS)</a></li><li><a href="/en-US/docs/Web/HTTP/Cross-Origin_Resource_Policy">Cross-Origin Resource Policy (CORP)</a></li><li><a href="/en-US/docs/Web/HTTP/Headers">Headers</a></li></ol></details></li><li class="section">References</li><li class="toggle"><details><summary>HTTP headers</summary><ol><li><a href="/en-US/docs/Web/HTTP/Headers/Accept"><code>Accept</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Accept-CH"><code>Accept-CH</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Accept-Encoding"><code>Accept-Encoding</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Accept-Language"><code>Accept-Language</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Accept-Patch"><code>Accept-Patch</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Accept-Post"><code>Accept-Post</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Accept-Ranges"><code>Accept-Ranges</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Credentials"><code>Access-Control-Allow-Credentials</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Headers"><code>Access-Control-Allow-Headers</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Methods"><code>Access-Control-Allow-Methods</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Origin"><code>Access-Control-Allow-Origin</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Access-Control-Expose-Headers"><code>Access-Control-Expose-Headers</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Access-Control-Max-Age"><code>Access-Control-Max-Age</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Access-Control-Request-Headers"><code>Access-Control-Request-Headers</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Access-Control-Request-Method"><code>Access-Control-Request-Method</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Age"><code>Age</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Allow"><code>Allow</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Alt-Svc"><code>Alt-Svc</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Alt-Used"><code>Alt-Used</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Attribution-Reporting-Eligible"><code>Attribution-Reporting-Eligible</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Attribution-Reporting-Register-Source"><code>Attribution-Reporting-Register-Source</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Attribution-Reporting-Register-Trigger"><code>Attribution-Reporting-Register-Trigger</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Authorization"><code>Authorization</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Cache-Control"><code>Cache-Control</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Clear-Site-Data"><code>Clear-Site-Data</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Connection"><code>Connection</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Digest"><code>Content-Digest</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Disposition"><code>Content-Disposition</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-DPR"><code>Content-DPR</code></a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Encoding"><code>Content-Encoding</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Language"><code>Content-Language</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Length"><code>Content-Length</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Location"><code>Content-Location</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Range"><code>Content-Range</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy"><code>Content-Security-Policy</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy-Report-Only"><code>Content-Security-Policy-Report-Only</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Type"><code>Content-Type</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Cookie"><code>Cookie</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Critical-CH"><code>Critical-CH</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Cross-Origin-Embedder-Policy"><code>Cross-Origin-Embedder-Policy</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Cross-Origin-Opener-Policy"><code>Cross-Origin-Opener-Policy</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Cross-Origin-Resource-Policy"><code>Cross-Origin-Resource-Policy</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Date"><code>Date</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Device-Memory"><code>Device-Memory</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/DNT"><code>DNT</code></a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Downlink"><code>Downlink</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/DPR"><code>DPR</code></a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Early-Data"><code>Early-Data</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/ECT"><code>ECT</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/ETag"><code>ETag</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Expect"><code>Expect</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Expect-CT"><code>Expect-CT</code></a><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Expires"><code>Expires</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Forwarded"><code>Forwarded</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/From"><code>From</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Host"><code>Host</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/If-Match"><code>If-Match</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/If-Modified-Since"><code>If-Modified-Since</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/If-None-Match"><code>If-None-Match</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/If-Range"><code>If-Range</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/If-Unmodified-Since"><code>If-Unmodified-Since</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Keep-Alive"><code>Keep-Alive</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Last-Modified"><code>Last-Modified</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Link"><code>Link</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Location"><code>Location</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Max-Forwards"><code>Max-Forwards</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/NEL"><code>NEL</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/No-Vary-Search"><code>No-Vary-Search</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Observe-Browsing-Topics"><code>Observe-Browsing-Topics</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Origin"><code>Origin</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Origin-Agent-Cluster"><code>Origin-Agent-Cluster</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy"><code>Permissions-Policy</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Pragma"><code>Pragma</code></a><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Priority"><code>Priority</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Proxy-Authenticate"><code>Proxy-Authenticate</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Proxy-Authorization"><code>Proxy-Authorization</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Range"><code>Range</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Referer"><code>Referer</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Referrer-Policy"><code>Referrer-Policy</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Refresh"><code>Refresh</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Report-To"><code>Report-To</code></a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Reporting-Endpoints"><code>Reporting-Endpoints</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Repr-Digest"><code>Repr-Digest</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Retry-After"><code>Retry-After</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/RTT"><code>RTT</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Save-Data"><code>Save-Data</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-Browsing-Topics"><code>Sec-Browsing-Topics</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-CH-Prefers-Color-Scheme"><code>Sec-CH-Prefers-Color-Scheme</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-CH-Prefers-Reduced-Motion"><code>Sec-CH-Prefers-Reduced-Motion</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-CH-Prefers-Reduced-Transparency"><code>Sec-CH-Prefers-Reduced-Transparency</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-CH-UA"><code>Sec-CH-UA</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Arch"><code>Sec-CH-UA-Arch</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Bitness"><code>Sec-CH-UA-Bitness</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Full-Version"><code>Sec-CH-UA-Full-Version</code></a><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Full-Version-List"><code>Sec-CH-UA-Full-Version-List</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Mobile"><code>Sec-CH-UA-Mobile</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Model"><code>Sec-CH-UA-Model</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Platform"><code>Sec-CH-UA-Platform</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Platform-Version"><code>Sec-CH-UA-Platform-Version</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-Fetch-Dest"><code>Sec-Fetch-Dest</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-Fetch-Mode"><code>Sec-Fetch-Mode</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-Fetch-Site"><code>Sec-Fetch-Site</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-Fetch-User"><code>Sec-Fetch-User</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-GPC"><code>Sec-GPC</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-Purpose"><code>Sec-Purpose</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-WebSocket-Accept"><code>Sec-WebSocket-Accept</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-WebSocket-Extensions"><code>Sec-WebSocket-Extensions</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-WebSocket-Key"><code>Sec-WebSocket-Key</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-WebSocket-Protocol"><code>Sec-WebSocket-Protocol</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Sec-WebSocket-Version"><code>Sec-WebSocket-Version</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Server"><code>Server</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Server-Timing"><code>Server-Timing</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Service-Worker"><code>Service-Worker</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Service-Worker-Allowed"><code>Service-Worker-Allowed</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Service-Worker-Navigation-Preload"><code>Service-Worker-Navigation-Preload</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Set-Cookie"><code>Set-Cookie</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Set-Login"><code>Set-Login</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/SourceMap"><code>SourceMap</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Speculation-Rules"><code>Speculation-Rules</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Strict-Transport-Security"><code>Strict-Transport-Security</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Supports-Loading-Mode"><code>Supports-Loading-Mode</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/TE"><code>TE</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Timing-Allow-Origin"><code>Timing-Allow-Origin</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Tk"><code>Tk</code></a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Trailer"><code>Trailer</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Transfer-Encoding"><code>Transfer-Encoding</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Upgrade"><code>Upgrade</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Upgrade-Insecure-Requests"><code>Upgrade-Insecure-Requests</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/User-Agent"><code>User-Agent</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Vary"><code>Vary</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Via"><code>Via</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Viewport-Width"><code>Viewport-Width</code></a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Want-Content-Digest"><code>Want-Content-Digest</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Want-Repr-Digest"><code>Want-Repr-Digest</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Warning"><code>Warning</code></a><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Width"><code>Width</code></a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/WWW-Authenticate"><code>WWW-Authenticate</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/X-Content-Type-Options"><code>X-Content-Type-Options</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/X-DNS-Prefetch-Control"><code>X-DNS-Prefetch-Control</code></a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/X-Forwarded-For"><code>X-Forwarded-For</code></a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/X-Forwarded-Host"><code>X-Forwarded-Host</code></a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/X-Forwarded-Proto"><code>X-Forwarded-Proto</code></a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/X-Frame-Options"><code>X-Frame-Options</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/X-Permitted-Cross-Domain-Policies"><code>X-Permitted-Cross-Domain-Policies</code></a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/X-Powered-By"><code>X-Powered-By</code></a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/X-Robots-Tag"><code>X-Robots-Tag</code></a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/X-XSS-Protection"><code>X-XSS-Protection</code></a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li></ol></details></li><li class="toggle"><details><summary>HTTP request methods</summary><ol><li><a href="/en-US/docs/Web/HTTP/Methods/CONNECT"><code>CONNECT</code></a></li><li><a href="/en-US/docs/Web/HTTP/Methods/DELETE"><code>DELETE</code></a></li><li><a href="/en-US/docs/Web/HTTP/Methods/GET"><code>GET</code></a></li><li><a href="/en-US/docs/Web/HTTP/Methods/HEAD"><code>HEAD</code></a></li><li><a href="/en-US/docs/Web/HTTP/Methods/OPTIONS"><code>OPTIONS</code></a></li><li><a href="/en-US/docs/Web/HTTP/Methods/PATCH"><code>PATCH</code></a></li><li><a href="/en-US/docs/Web/HTTP/Methods/POST"><code>POST</code></a></li><li><a href="/en-US/docs/Web/HTTP/Methods/PUT"><code>PUT</code></a></li><li><a href="/en-US/docs/Web/HTTP/Methods/TRACE"><code>TRACE</code></a></li></ol></details></li><li class="toggle"><details><summary>HTTP response status codes</summary><ol><li><a href="/en-US/docs/Web/HTTP/Status/100"><code>100 Continue</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/101"><code>101 Switching Protocols</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/102"><code>102 Processing</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/103"><code>103 Early Hints</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/200"><code>200 OK</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/201"><code>201 Created</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/202"><code>202 Accepted</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/203"><code>203 Non-Authoritative Information</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/204"><code>204 No Content</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/205"><code>205 Reset Content</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/206"><code>206 Partial Content</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/207"><code>207 Multi-Status</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/208"><code>208 Already Reported</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/226"><code>226 IM Used</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/300"><code>300 Multiple Choices</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/301"><code>301 Moved Permanently</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/302"><code>302 Found</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/303"><code>303 See Other</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/304"><code>304 Not Modified</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/307"><code>307 Temporary Redirect</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/308"><code>308 Permanent Redirect</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/400"><code>400 Bad Request</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/401"><code>401 Unauthorized</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/402"><code>402 Payment Required</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/403"><code>403 Forbidden</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/404"><code>404 Not Found</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/405"><code>405 Method Not Allowed</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/406"><code>406 Not Acceptable</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/407"><code>407 Proxy Authentication Required</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/408"><code>408 Request Timeout</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/409"><code>409 Conflict</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/410"><code>410 Gone</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/411"><code>411 Length Required</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/412"><code>412 Precondition Failed</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/413"><code>413 Content Too Large</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/414"><code>414 URI Too Long</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/415"><code>415 Unsupported Media Type</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/416"><code>416 Range Not Satisfiable</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/417"><code>417 Expectation Failed</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/418"><code>418 I'm a teapot</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/421"><code>421 Misdirected Request</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/422"><code>422 Unprocessable Content</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/423"><code>423 Locked</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/424"><code>424 Failed Dependency</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/425"><code>425 Too Early</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/426"><code>426 Upgrade Required</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/428"><code>428 Precondition Required</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/429"><code>429 Too Many Requests</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/431"><code>431 Request Header Fields Too Large</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/451"><code>451 Unavailable For Legal Reasons</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/500"><code>500 Internal Server Error</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/501"><code>501 Not Implemented</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/502"><code>502 Bad Gateway</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/503"><code>503 Service Unavailable</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/504"><code>504 Gateway Timeout</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/505"><code>505 HTTP Version Not Supported</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/506"><code>506 Variant Also Negotiates</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/507"><code>507 Insufficient Storage</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/508"><code>508 Loop Detected</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/510"><code>510 Not Extended</code></a></li><li><a href="/en-US/docs/Web/HTTP/Status/511"><code>511 Network Authentication Required</code></a></li></ol></details></li><li class="toggle"><details><summary>CSP directives</summary><ol><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/base-uri"><code>CSP: base-uri</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/block-all-mixed-content"><code>CSP: block-all-mixed-content</code></a><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/child-src"><code>CSP: child-src</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/connect-src"><code>CSP: connect-src</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/default-src"><code>CSP: default-src</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/fenced-frame-src"><code>CSP: fenced-frame-src</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/font-src"><code>CSP: font-src</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/form-action"><code>CSP: form-action</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/frame-ancestors"><code>CSP: frame-ancestors</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/frame-src"><code>CSP: frame-src</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/img-src"><code>CSP: img-src</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/manifest-src"><code>CSP: manifest-src</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/media-src"><code>CSP: media-src</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/object-src"><code>CSP: object-src</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/prefetch-src"><code>CSP: prefetch-src</code></a><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/report-to"><code>CSP: report-to</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/report-uri"><code>CSP: report-uri</code></a><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/require-trusted-types-for"><code>CSP: require-trusted-types-for</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/sandbox"><code>CSP: sandbox</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src"><code>CSP: script-src</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src-attr"><code>CSP: script-src-attr</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src-elem"><code>CSP: script-src-elem</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/style-src"><code>CSP: style-src</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/style-src-attr"><code>CSP: style-src-attr</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/style-src-elem"><code>CSP: style-src-elem</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/trusted-types"><code>CSP: trusted-types</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/upgrade-insecure-requests"><code>CSP: upgrade-insecure-requests</code></a></li><li><a href="/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/worker-src"><code>CSP: worker-src</code></a></li></ol></details></li><li class="toggle"><details><summary>CORS errors</summary><ol><li><a href="/en-US/docs/Web/HTTP/CORS/Errors/CORSDisabled"><code>Reason: CORS disabled</code></a></li><li><a href="/en-US/docs/Web/HTTP/CORS/Errors/CORSAllowOriginNotMatchingOrigin"><code>Reason: CORS header 'Access-Control-Allow-Origin' does not match 'xyz'</code></a></li><li><a href="/en-US/docs/Web/HTTP/CORS/Errors/CORSMissingAllowOrigin"><code>Reason: CORS header 'Access-Control-Allow-Origin' missing</code></a></li><li><a href="/en-US/docs/Web/HTTP/CORS/Errors/CORSOriginHeaderNotAdded"><code>Reason: CORS header 'Origin' cannot be added</code></a></li><li><a href="/en-US/docs/Web/HTTP/CORS/Errors/CORSPreflightDidNotSucceed"><code>Reason: CORS preflight channel did not succeed</code></a></li><li><a href="/en-US/docs/Web/HTTP/CORS/Errors/CORSDidNotSucceed"><code>Reason: CORS request did not succeed</code></a></li><li><a href="/en-US/docs/Web/HTTP/CORS/Errors/CORSExternalRedirectNotAllowed"><code>Reason: CORS request external redirect not allowed</code></a></li><li><a href="/en-US/docs/Web/HTTP/CORS/Errors/CORSRequestNotHttp"><code>Reason: CORS request not HTTP</code></a></li><li><a href="/en-US/docs/Web/HTTP/CORS/Errors/CORSNotSupportingCredentials"><code>Reason: Credential is not supported if the CORS header 'Access-Control-Allow-Origin' is '*'</code></a></li><li><a href="/en-US/docs/Web/HTTP/CORS/Errors/CORSMethodNotFound"><code>Reason: Did not find method in CORS header 'Access-Control-Allow-Methods'</code></a></li><li><a href="/en-US/docs/Web/HTTP/CORS/Errors/CORSMIssingAllowCredentials"><code>Reason: expected 'true' in CORS header 'Access-Control-Allow-Credentials'</code></a></li><li><a href="/en-US/docs/Web/HTTP/CORS/Errors/CORSInvalidAllowHeader"><code>Reason: invalid token 'xyz' in CORS header 'Access-Control-Allow-Headers'</code></a></li><li><a href="/en-US/docs/Web/HTTP/CORS/Errors/CORSInvalidAllowMethod"><code>Reason: invalid token 'xyz' in CORS header 'Access-Control-Allow-Methods'</code></a></li><li><a href="/en-US/docs/Web/HTTP/CORS/Errors/CORSMissingAllowHeaderFromPreflight"><code>Reason: missing token 'xyz' in CORS header 'Access-Control-Allow-Headers' from CORS preflight channel</code></a></li><li><a href="/en-US/docs/Web/HTTP/CORS/Errors/CORSMultipleAllowOriginNotAllowed"><code>Reason: Multiple CORS header 'Access-Control-Allow-Origin' not allowed</code></a></li></ol></details></li><li class="toggle"><details><summary>Permissions-Policy directives</summary><ol><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/accelerometer"><code>Permissions-Policy: accelerometer</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/ambient-light-sensor"><code>Permissions-Policy: ambient-light-sensor</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/attribution-reporting"><code>Permissions-Policy: attribution-reporting</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/autoplay"><code>Permissions-Policy: autoplay</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/bluetooth"><code>Permissions-Policy: bluetooth</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/browsing-topics"><code>Permissions-Policy: browsing-topics</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr><abbr class="icon icon-nonstandard" title="Non-standard. Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/camera"><code>Permissions-Policy: camera</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/compute-pressure"><code>Permissions-Policy: compute-pressure</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/cross-origin-isolated"><code>Permissions-Policy: cross-origin-isolated</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/display-capture"><code>Permissions-Policy: display-capture</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/document-domain"><code>Permissions-Policy: document-domain</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/encrypted-media"><code>Permissions-Policy: encrypted-media</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/fullscreen"><code>Permissions-Policy: fullscreen</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/gamepad"><code>Permissions-Policy: gamepad</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/geolocation"><code>Permissions-Policy: geolocation</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/gyroscope"><code>Permissions-Policy: gyroscope</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/hid"><code>Permissions-Policy: hid</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/identity-credentials-get"><code>Permissions-Policy: identity-credentials-get</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/idle-detection"><code>Permissions-Policy: idle-detection</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/local-fonts"><code>Permissions-Policy: local-fonts</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/magnetometer"><code>Permissions-Policy: magnetometer</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/microphone"><code>Permissions-Policy: microphone</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/midi"><code>Permissions-Policy: midi</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/otp-credentials"><code>Permissions-Policy: otp-credentials</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/payment"><code>Permissions-Policy: payment</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/picture-in-picture"><code>Permissions-Policy: picture-in-picture</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/publickey-credentials-create"><code>Permissions-Policy: publickey-credentials-create</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/publickey-credentials-get"><code>Permissions-Policy: publickey-credentials-get</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/screen-wake-lock"><code>Permissions-Policy: screen-wake-lock</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/serial"><code>Permissions-Policy: serial</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/speaker-selection"><code>Permissions-Policy: speaker-selection</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/storage-access"><code>Permissions-Policy: storage-access</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/usb"><code>Permissions-Policy: usb</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/web-share"><code>Permissions-Policy: web-share</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/window-management"><code>Permissions-Policy: window-management</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li><li><a href="/en-US/docs/Web/HTTP/Headers/Permissions-Policy/xr-spatial-tracking"><code>Permissions-Policy: xr-spatial-tracking</code></a><abbr class="icon icon-experimental" title="Experimental. Expect behavior to change in the future."> <span class="visually-hidden">Experimental</span> </abbr></li></ol></details></li><li><a href="/en-US/docs/Web/HTTP/Resources_and_specifications">HTTP resources and specifications</a></li></ol></div></div><section class="place side"></section></nav></aside><div class="toc-container"><aside class="toc"><nav><div class="document-toc-container"><section class="document-toc"><header><h2 class="document-toc-heading">In this article</h2></header><ul class="document-toc-list"><li class="document-toc-item "><a class="document-toc-link" href="#invention_of_the_world_wide_web">Invention of the World Wide Web</a></li><li class="document-toc-item "><a class="document-toc-link" href="#http0.9_–_the_one-line_protocol">HTTP/0.9 – The one-line protocol</a></li><li class="document-toc-item "><a class="document-toc-link" href="#http1.0_–_building_extensibility">HTTP/1.0 – Building extensibility</a></li><li class="document-toc-item "><a class="document-toc-link" href="#http1.1_–_the_standardized_protocol">HTTP/1.1 – The standardized protocol</a></li><li class="document-toc-item "><a class="document-toc-link" href="#more_than_two_decades_of_development">More than two decades of development</a></li><li class="document-toc-item "><a class="document-toc-link" href="#http2_–_a_protocol_for_greater_performance">HTTP/2 – A protocol for greater performance</a></li><li class="document-toc-item "><a class="document-toc-link" href="#post-http2_evolution">Post-HTTP/2 evolution</a></li><li class="document-toc-item "><a class="document-toc-link" href="#http3_-_http_over_quic">HTTP/3 - HTTP over QUIC</a></li><li class="document-toc-item "><a class="document-toc-link" href="#see_also">See also</a></li></ul></section></div></nav></aside><section class="place side"></section></div></div><main id="content" class="main-content "><article class="main-page-content" lang="en-US"><header><h1>Evolution of HTTP</h1></header><div class="section-content"><p><strong>HTTP</strong> (HyperText Transfer Protocol) is the underlying protocol of the World Wide Web. Developed by Tim Berners-Lee and his team between 1989-1991, HTTP has gone through many changes that have helped maintain its simplicity while shaping its flexibility. Keep reading to learn how HTTP evolved from a protocol designed to exchange files in a semitrusted laboratory environment into a modern internet maze that carries images and videos in high resolution and 3D.</p></div><section aria-labelledby="invention_of_the_world_wide_web"><h2 id="invention_of_the_world_wide_web"><a href="#invention_of_the_world_wide_web">Invention of the World Wide Web</a></h2><div class="section-content"><p>In 1989, while working at CERN, Tim Berners-Lee wrote a proposal to build a hypertext system over the internet. Initially called the <em>Mesh</em>, it was later renamed the <em>World Wide Web</em> during its implementation in 1990. Built over the existing TCP and IP protocols, it consisted of 4 building blocks:</p> <ul> <li>A textual format to represent hypertext documents, the <em><a href="/en-US/docs/Web/HTML">HyperText Markup Language</a></em> (HTML).</li> <li>A simple protocol to exchange these documents, the <em>HyperText Transfer Protocol</em> (HTTP).</li> <li>A client to display (and edit) these documents, the first web browser called the <em>WorldWideWeb</em>.</li> <li>A server to give access to the document, an early version of <em>httpd</em>.</li> </ul> <p>These four building blocks were completed by the end of 1990, and the first servers were running outside of CERN by early 1991. On August 6, 1991, Tim Berners-Lee <a href="https://www.w3.org/People/Berners-Lee/1991/08/art-6484.txt" class="external" target="_blank">posted</a> on the public <em>alt.hypertext</em> newsgroup. This is now considered to be the official start of the World Wide Web as a public project.</p> <p>The HTTP protocol used in those early phases was very simple. It was later dubbed HTTP/0.9 and is sometimes called the one-line protocol.</p></div></section><section aria-labelledby="http0.9_–_the_one-line_protocol"><h2 id="http0.9_–_the_one-line_protocol"><a href="#http0.9_–_the_one-line_protocol">HTTP/0.9 – The one-line protocol</a></h2><div class="section-content"><p>The initial version of HTTP had no version number; it was later called 0.9 to differentiate it from later versions. HTTP/0.9 was extremely simple: requests consisted of a single line and started with the only possible method <a href="/en-US/docs/Web/HTTP/Methods/GET"><code>GET</code></a> followed by the path to the resource. The full URL wasn't included as the protocol, server, and port weren't necessary once connected to the server.</p> <div class="code-example"><div class="example-header"><span class="language-name">http</span></div><pre class="brush: http notranslate"><code>GET /my-page.html </code></pre></div> <p>The response was extremely simple, too: it only consisted of the file itself.</p> <div class="code-example"><div class="example-header"><span class="language-name">html</span></div><pre class="brush: html notranslate"><code><html> A very simple HTML page </html> </code></pre></div> <p>Unlike subsequent evolutions, there were no HTTP headers. This meant that only HTML files could be transmitted. There were no status or error codes. If there was a problem, a specific HTML file was generated and included a description of the problem for human consumption.</p></div></section><section aria-labelledby="http1.0_–_building_extensibility"><h2 id="http1.0_–_building_extensibility"><a href="#http1.0_–_building_extensibility">HTTP/1.0 – Building extensibility</a></h2><div class="section-content"><p>HTTP/0.9 was very limited, but browsers and servers quickly made it more versatile:</p> <ul> <li>Versioning information was sent within each request (<code>HTTP/1.0</code> was appended to the <code>GET</code> line).</li> <li>A status code line was also sent at the beginning of a response. This allowed the browser itself to recognize the success or failure of a request and adapt its behavior accordingly. For example, updating or using its local cache in a specific way.</li> <li>The concept of HTTP headers was introduced for both requests and responses. Metadata could be transmitted and the protocol became extremely flexible and extensible.</li> <li>Documents other than plain HTML files could be transmitted thanks to the <a href="/en-US/docs/Web/HTTP/Headers/Content-Type"><code>Content-Type</code></a> header.</li> </ul> <p>At this point in time, a typical request and response looked like this:</p> <div class="code-example"><div class="example-header"><span class="language-name">http</span></div><pre class="brush: http notranslate"><code>GET /my-page.html HTTP/1.0 User-Agent: NCSA_Mosaic/2.0 (Windows 3.1) HTTP/1.0 200 OK Date: Tue, 15 Nov 1994 08:12:31 GMT Server: CERN/3.0 libwww/2.17 Content-Type: text/html <HTML> A page with an image <IMG SRC="/my-image.gif"> </HTML> </code></pre></div> <p>It was followed by a second connection and a request to fetch the image (with the corresponding response):</p> <div class="code-example"><div class="example-header"><span class="language-name">http</span></div><pre class="brush: http notranslate"><code>GET /my-image.gif HTTP/1.0 User-Agent: NCSA_Mosaic/2.0 (Windows 3.1) HTTP/1.0 200 OK Date: Tue, 15 Nov 1994 08:12:32 GMT Server: CERN/3.0 libwww/2.17 Content-Type: text/gif (image content) </code></pre></div> <p>Between 1991-1995, these were introduced with a try-and-see approach. A server and a browser would add a feature and see if it got traction. Interoperability problems were common. In an effort to solve these issues, an informational document that described the common practices was published in November 1996. This was known as <a href="https://datatracker.ietf.org/doc/html/rfc1945" class="external" target="_blank">RFC 1945</a> and defined HTTP/1.0.</p></div></section><section aria-labelledby="http1.1_–_the_standardized_protocol"><h2 id="http1.1_–_the_standardized_protocol"><a href="#http1.1_–_the_standardized_protocol">HTTP/1.1 – The standardized protocol</a></h2><div class="section-content"><p>In the meantime, proper standardization was in progress. This happened in parallel to the diverse implementations of HTTP/1.0. The first standardized version of HTTP, HTTP/1.1, was published in early 1997, only a few months after HTTP/1.0.</p> <p>HTTP/1.1 clarified ambiguities and introduced numerous improvements:</p> <ul> <li>A connection could be reused, which saved time. It no longer needed to be opened multiple times to display the resources embedded in the single original document.</li> <li>Pipelining was added. This allowed a second request to be sent before the answer to the first one was fully transmitted. This lowered the latency of the communication.</li> <li>Chunked responses were also supported.</li> <li>Additional cache control mechanisms were introduced.</li> <li>Content negotiation, including language, encoding, and type, was introduced. A client and a server could now agree on which content to exchange.</li> <li>Thanks to the <a href="/en-US/docs/Web/HTTP/Headers/Host"><code>Host</code></a> header, the ability to host different domains from the same IP address allowed server collocation.</li> </ul> <p>A typical flow of requests, all through one single connection, looked like this:</p> <div class="code-example"><div class="example-header"><span class="language-name">http</span></div><pre class="brush: http notranslate"><code>GET /en-US/docs/Glossary/CORS-safelisted_request_header HTTP/1.1 Host: developer.mozilla.org User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:50.0) Gecko/20100101 Firefox/50.0 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language: en-US,en;q=0.5 Accept-Encoding: gzip, deflate, br Referer: https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted_request_header HTTP/1.1 200 OK Connection: Keep-Alive Content-Encoding: gzip Content-Type: text/html; charset=utf-8 Date: Wed, 20 Jul 2016 10:55:30 GMT Etag: "547fa7e369ef56031dd3bff2ace9fc0832eb251a" Keep-Alive: timeout=5, max=1000 Last-Modified: Tue, 19 Jul 2016 00:59:33 GMT Server: Apache Transfer-Encoding: chunked Vary: Cookie, Accept-Encoding (content) GET /static/img/header-background.png HTTP/1.1 Host: developer.mozilla.org User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:50.0) Gecko/20100101 Firefox/50.0 Accept: */* Accept-Language: en-US,en;q=0.5 Accept-Encoding: gzip, deflate, br Referer: https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted_request_header HTTP/1.1 200 OK Age: 9578461 Cache-Control: public, max-age=315360000 Connection: keep-alive Content-Length: 3077 Content-Type: image/png Date: Thu, 31 Mar 2016 13:34:46 GMT Last-Modified: Wed, 21 Oct 2015 18:27:50 GMT Server: Apache (image content of 3077 bytes) </code></pre></div> <p>HTTP/1.1 was first published as <a href="https://datatracker.ietf.org/doc/html/rfc2068" class="external" target="_blank">RFC 2068</a> in January 1997.</p></div></section><section aria-labelledby="more_than_two_decades_of_development"><h2 id="more_than_two_decades_of_development"><a href="#more_than_two_decades_of_development">More than two decades of development</a></h2><div class="section-content"><p>The extensibility of HTTP made it easy to create new headers and methods. Even though the HTTP/1.1 protocol was refined over two revisions, <a href="https://datatracker.ietf.org/doc/html/rfc2616" class="external" target="_blank">RFC 2616</a> published in June 1999 and <a href="https://datatracker.ietf.org/doc/html/rfc7230" class="external" target="_blank">RFC 7230</a>-<a href="https://datatracker.ietf.org/doc/html/rfc7235" class="external" target="_blank">RFC 7235</a> published in June 2014 before the release of HTTP/2, it was extremely stable for more than 15 years. HTTP/1.1 was updated again in 2022 with <a href="https://datatracker.ietf.org/doc/html/rfc9110" class="external" target="_blank">RFC 9110</a>. Not only was HTTP/1.1 updated, but all of HTTP was revised and is now split into the following documents: semantics (<a href="https://datatracker.ietf.org/doc/html/rfc9110" class="external" target="_blank">RFC 9110</a>), caching (<a href="https://datatracker.ietf.org/doc/html/rfc9111" class="external" target="_blank">RFC 9111</a>) applying to all HTTP versions, and HTTP/1.1 (<a href="https://datatracker.ietf.org/doc/html/rfc9112" class="external" target="_blank">RFC 9112</a>), HTTP/2 (<a href="https://datatracker.ietf.org/doc/html/rfc9113" class="external" target="_blank">RFC 9113</a>), and HTTP/3 (<a href="https://datatracker.ietf.org/doc/html/rfc9114" class="external" target="_blank">RFC 9114</a>). In addition, the specification finally achieved the status of Internet Standard (STD 97), whereas before it was always a proposed/draft standard.</p></div></section><section aria-labelledby="using_http_for_secure_transmissions"><h3 id="using_http_for_secure_transmissions"><a href="#using_http_for_secure_transmissions">Using HTTP for secure transmissions</a></h3><div class="section-content"><p>The largest change to HTTP was made at the end of 1994. Instead of sending HTTP over a basic TCP/IP stack, the computer-services company Netscape Communications created an additional encrypted transmission layer on top of it: SSL. SSL 1.0 was never released to the public, but SSL 2.0 and its successor SSL 3.0 allowed for the creation of e-commerce websites. To do this, they encrypted and guaranteed the authenticity of the messages exchanged between the server and client. SSL was eventually standardized and became TLS.</p> <p>During the same time period, it became clear that an encrypted transport layer was needed. The web was no longer a mostly academic network, and instead became a jungle where advertisers, random individuals, and criminals competed for as much private data as possible. As the applications built over HTTP became more powerful and required access to private information like address books, email, and user location, TLS became necessary outside the e-commerce use case.</p></div></section><section aria-labelledby="using_http_for_complex_applications"><h3 id="using_http_for_complex_applications"><a href="#using_http_for_complex_applications">Using HTTP for complex applications</a></h3><div class="section-content"><p>Tim Berners-Lee didn't originally envision HTTP as a read-only medium. He wanted to create a web where people could add and move documents remotely—a kind of distributed file system. Around 1996, HTTP was extended to allow authoring, and a standard called WebDAV was created. It grew to include specific applications like CardDAV for handling address book entries and CalDAV for dealing with calendars. But all these *DAV extensions had a flaw: they were only usable when implemented by the servers.</p> <p>In 2000, a new pattern for using HTTP was designed: <a href="/en-US/docs/Glossary/REST">representational state transfer</a> (or REST). The API wasn't based on the new HTTP methods, but instead relied on access to specific URIs with basic HTTP/1.1 methods. This allowed any web application to let an API retrieve and modify its data without having to update the browsers or the servers. All necessary information was embedded in the files that the websites served through standard HTTP/1.1. The drawback of the REST model was that each website defined its own nonstandard RESTful API and had total control of it. This differed from the *DAV extensions where clients and servers were interoperable. RESTful APIs became very common in the 2010s.</p> <p>Since 2005, more APIs have become available to web pages. Several of these APIs create extensions to the HTTP protocol for specific purposes:</p> <ul> <li><a href="/en-US/docs/Web/API/Server-sent_events">Server-sent events</a>, where the server can push occasional messages to the browser.</li> <li><a href="/en-US/docs/Web/API/WebSockets_API">WebSocket</a>, a new protocol that can be set up by upgrading an existing HTTP connection.</li> </ul></div></section><section aria-labelledby="relaxing_the_security-model_of_the_web"><h3 id="relaxing_the_security-model_of_the_web"><a href="#relaxing_the_security-model_of_the_web">Relaxing the security-model of the web</a></h3><div class="section-content"><p>HTTP is independent of the web security model, known as the <a href="/en-US/docs/Web/Security/Same-origin_policy">same-origin policy</a>. In fact, the current web security model was developed after the creation of HTTP! Over the years, it proved useful to lift some restrictions of this policy under certain constraints. The server transmitted how much and when to lift such restrictions to the client using a new set of HTTP headers. These were defined in specifications like <a href="/en-US/docs/Glossary/CORS">Cross-Origin Resource Sharing</a> (CORS) and the <a href="/en-US/docs/Web/HTTP/CSP">Content Security Policy</a> (CSP).</p> <p>In addition to these large extensions, many other headers were added, sometimes only experimentally. Notable headers are the Do Not Track (<a href="/en-US/docs/Web/HTTP/Headers/DNT"><code>DNT</code></a>) header to control privacy, <a href="/en-US/docs/Web/HTTP/Headers/X-Frame-Options"><code>X-Frame-Options</code></a>, and <a href="/en-US/docs/Web/HTTP/Headers/Upgrade-Insecure-Requests"><code>Upgrade-Insecure-Requests</code></a> but many more exist.</p></div></section><section aria-labelledby="http2_–_a_protocol_for_greater_performance"><h2 id="http2_–_a_protocol_for_greater_performance"><a href="#http2_–_a_protocol_for_greater_performance">HTTP/2 – A protocol for greater performance</a></h2><div class="section-content"><p>Over the years, web pages became more complex. Some of them were even applications in their own right. More visual media was displayed and the volume and size of scripts adding interactivity also increased. Much more data was transmitted over significantly more HTTP requests and this created more complexity and overhead for HTTP/1.1 connections. To account for this, Google implemented an experimental protocol SPDY in the early 2010s. This alternative way of exchanging data between client and server amassed interest from developers working on both browsers and servers. SPDY defined an increase in responsiveness and solved the problem of duplicate data transmission, serving as the foundation for the HTTP/2 protocol.</p> <p>The HTTP/2 protocol differs from HTTP/1.1 in a few ways:</p> <ul> <li>It's a binary protocol rather than a text protocol. It can't be read and created manually. Despite this hurdle, it allows for the implementation of improved optimization techniques.</li> <li>It's a multiplexed protocol. Parallel requests can be made over the same connection, removing the constraints of the HTTP/1.x protocol.</li> <li>It compresses headers. As these are often similar among a set of requests, this removes the duplication and overhead of data transmitted.</li> </ul> <p>Officially standardized in May 2015, HTTP/2 use peaked in January 2022 at 46.9% of all websites (see <a href="https://w3techs.com/technologies/details/ce-http2" class="external" target="_blank">these stats</a>). High-traffic websites showed the most rapid adoption in an effort to save on data transfer overhead and subsequent budgets.</p> <p>This rapid adoption was likely because HTTP/2 didn't require changes to websites and applications. To use it, only an up-to-date server that communicated with a recent browser was necessary. Only a limited set of groups was needed to trigger adoption, and as legacy browser and server versions were renewed, usage was naturally increased, without significant work for web developers.</p></div></section><section aria-labelledby="post-http2_evolution"><h2 id="post-http2_evolution"><a href="#post-http2_evolution">Post-HTTP/2 evolution</a></h2><div class="section-content"><p>HTTP's extensibility is still being used to add new features. Notably, we can cite new extensions of the HTTP protocol that appeared in 2016:</p> <ul> <li>Support for <a href="/en-US/docs/Web/HTTP/Headers/Alt-Svc"><code>Alt-Svc</code></a> allowed the dissociation of the identification and the location of a given resource. This meant a smarter <a href="/en-US/docs/Glossary/CDN">CDN</a> caching mechanism.</li> <li>The introduction of <a href="/en-US/docs/Web/HTTP/Client_hints">client hints</a> allowed the browser or client to proactively communicate information about its requirements and hardware constraints to the server.</li> <li>The introduction of security-related prefixes in the <a href="/en-US/docs/Web/HTTP/Headers/Cookie"><code>Cookie</code></a> header helped guarantee that secure cookies couldn't be altered.</li> </ul></div></section><section aria-labelledby="http3_-_http_over_quic"><h2 id="http3_-_http_over_quic"><a href="#http3_-_http_over_quic">HTTP/3 - HTTP over QUIC</a></h2><div class="section-content"><p>The next major version of HTTP, HTTP/3 has the same semantics as earlier versions of HTTP but uses <a href="/en-US/docs/Glossary/QUIC">QUIC</a> instead of <a href="/en-US/docs/Glossary/TCP">TCP</a> for the transport layer portion. By October 2022, <a href="https://w3techs.com/technologies/details/ce-http3" class="external" target="_blank">26% of all websites were using HTTP/3</a>.</p> <p>QUIC is designed to provide much lower latency for HTTP connections. Like HTTP/2, it is a multiplexed protocol, but HTTP/2 runs over a single TCP connection, so packet loss detection and retransmission handled at the TCP layer can block all streams. QUIC runs multiple streams over <a href="/en-US/docs/Glossary/UDP">UDP</a> and implements packet loss detection and retransmission independently for each stream, so that if an error occurs, only the stream with data in that packet is blocked.</p> <p>Defined in <a href="https://datatracker.ietf.org/doc/html/rfc9114" class="external" target="_blank">RFC 9114</a>, <a href="https://caniuse.com/http3" class="external" target="_blank">HTTP/3 is supported by most major browsers</a> including Chromium (and its variants such as Chrome and Edge) and Firefox.</p></div></section><section aria-labelledby="see_also"><h2 id="see_also"><a href="#see_also">See also</a></h2><div class="section-content"><ul> <li><a href="/en-US/docs/Web/HTTP/Connection_management_in_HTTP_1.x">Connection management in HTTP/1.x</a></li> <li><a href="/en-US/docs/Web/HTTP/Protocol_upgrade_mechanism">Protocol upgrade mechanism</a></li> <li><a href="/en-US/docs/Web/HTTP/Resources_and_specifications">HTTP resources and specifications</a></li> <li>Glossary terms: <ul> <li><a href="/en-US/docs/Glossary/HTTP">HTTP</a></li> <li><a href="/en-US/docs/Glossary/HTTP_2">HTTP/2</a></li> <li><a href="/en-US/docs/Glossary/QUIC">QUIC</a></li> <li><a href="/en-US/docs/Glossary/Round_Trip_Time">Round Trip Time (RTT)</a></li> <li><a href="/en-US/docs/Glossary/TCP_slow_start">TCP slow start</a></li> <li><a href="/en-US/docs/Glossary/TCP">Transmission Control Protocol (TCP)</a></li> </ul> </li> </ul></div></section></article><aside class="article-footer"><div class="article-footer-inner"><div class="svg-container"><svg xmlns="http://www.w3.org/2000/svg" width="162" height="162" viewBox="0 0 162 162" fill="none" role="none"><mask id="b" fill="#fff"><path d="M97.203 47.04c8.113-7.886 18.004-13.871 28.906-17.492a78 78 0 0 1 33.969-3.39c11.443 1.39 22.401 5.295 32.024 11.411s17.656 14.28 23.476 23.86c5.819 9.579 9.269 20.318 10.083 31.385a69.85 69.85 0 0 1-5.387 32.44c-4.358 10.272-11.115 19.443-19.747 26.801-8.632 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Content available under<!-- --> <a href="/en-US/docs/MDN/Writing_guidelines/Attrib_copyright_license">a Creative Commons license</a>.</p></div></div></footer></div><script type="application/json" id="hydration">{"url":"/en-US/docs/Web/HTTP/Evolution_of_HTTP","doc":{"body":[{"type":"prose","value":{"id":null,"title":null,"isH3":false,"content":"<p><strong>HTTP</strong> (HyperText Transfer Protocol) is the underlying protocol of the World Wide Web. Developed by Tim Berners-Lee and his team between 1989-1991, HTTP has gone through many changes that have helped maintain its simplicity while shaping its flexibility. Keep reading to learn how HTTP evolved from a protocol designed to exchange files in a semitrusted laboratory environment into a modern internet maze that carries images and videos in high resolution and 3D.</p>"}},{"type":"prose","value":{"id":"invention_of_the_world_wide_web","title":"Invention of the World Wide Web","isH3":false,"content":"<p>In 1989, while working at CERN, Tim Berners-Lee wrote a proposal to build a hypertext system over the internet. Initially called the <em>Mesh</em>, it was later renamed the <em>World Wide Web</em> during its implementation in 1990. Built over the existing TCP and IP protocols, it consisted of 4 building blocks:</p>\n<ul>\n<li>A textual format to represent hypertext documents, the <em><a href=\"/en-US/docs/Web/HTML\">HyperText Markup Language</a></em> (HTML).</li>\n<li>A simple protocol to exchange these documents, the <em>HyperText Transfer Protocol</em> (HTTP).</li>\n<li>A client to display (and edit) these documents, the first web browser called the <em>WorldWideWeb</em>.</li>\n<li>A server to give access to the document, an early version of <em>httpd</em>.</li>\n</ul>\n<p>These four building blocks were completed by the end of 1990, and the first servers were running outside of CERN by early 1991. On August 6, 1991, Tim Berners-Lee <a href=\"https://www.w3.org/People/Berners-Lee/1991/08/art-6484.txt\" class=\"external\" target=\"_blank\">posted</a> on the public <em>alt.hypertext</em> newsgroup. This is now considered to be the official start of the World Wide Web as a public project.</p>\n<p>The HTTP protocol used in those early phases was very simple. It was later dubbed HTTP/0.9 and is sometimes called the one-line protocol.</p>"}},{"type":"prose","value":{"id":"http0.9_–_the_one-line_protocol","title":"HTTP/0.9 – The one-line protocol","isH3":false,"content":"<p>The initial version of HTTP had no version number; it was later called 0.9 to differentiate it from later versions. HTTP/0.9 was extremely simple: requests consisted of a single line and started with the only possible method <a href=\"/en-US/docs/Web/HTTP/Methods/GET\"><code>GET</code></a> followed by the path to the resource. The full URL wasn't included as the protocol, server, and port weren't necessary once connected to the server.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">http</span></div><pre class=\"brush: http notranslate\"><code>GET /my-page.html\n</code></pre></div>\n<p>The response was extremely simple, too: it only consisted of the file itself.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">html</span></div><pre class=\"brush: html notranslate\"><code><html>\n A very simple HTML page\n</html>\n</code></pre></div>\n<p>Unlike subsequent evolutions, there were no HTTP headers. This meant that only HTML files could be transmitted. There were no status or error codes. If there was a problem, a specific HTML file was generated and included a description of the problem for human consumption.</p>"}},{"type":"prose","value":{"id":"http1.0_–_building_extensibility","title":"HTTP/1.0 – Building extensibility","isH3":false,"content":"<p>HTTP/0.9 was very limited, but browsers and servers quickly made it more versatile:</p>\n<ul>\n<li>Versioning information was sent within each request (<code>HTTP/1.0</code> was appended to the <code>GET</code> line).</li>\n<li>A status code line was also sent at the beginning of a response. This allowed the browser itself to recognize the success or failure of a request and adapt its behavior accordingly. For example, updating or using its local cache in a specific way.</li>\n<li>The concept of HTTP headers was introduced for both requests and responses. Metadata could be transmitted and the protocol became extremely flexible and extensible.</li>\n<li>Documents other than plain HTML files could be transmitted thanks to the <a href=\"/en-US/docs/Web/HTTP/Headers/Content-Type\"><code>Content-Type</code></a> header.</li>\n</ul>\n<p>At this point in time, a typical request and response looked like this:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">http</span></div><pre class=\"brush: http notranslate\"><code>GET /my-page.html HTTP/1.0\nUser-Agent: NCSA_Mosaic/2.0 (Windows 3.1)\n\nHTTP/1.0 200 OK\nDate: Tue, 15 Nov 1994 08:12:31 GMT\nServer: CERN/3.0 libwww/2.17\nContent-Type: text/html\n<HTML>\nA page with an image\n <IMG SRC=\"/my-image.gif\">\n</HTML>\n</code></pre></div>\n<p>It was followed by a second connection and a request to fetch the image (with the corresponding response):</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">http</span></div><pre class=\"brush: http notranslate\"><code>GET /my-image.gif HTTP/1.0\nUser-Agent: NCSA_Mosaic/2.0 (Windows 3.1)\n\nHTTP/1.0 200 OK\nDate: Tue, 15 Nov 1994 08:12:32 GMT\nServer: CERN/3.0 libwww/2.17\nContent-Type: text/gif\n(image content)\n</code></pre></div>\n<p>Between 1991-1995, these were introduced with a try-and-see approach. A server and a browser would add a feature and see if it got traction. Interoperability problems were common. In an effort to solve these issues, an informational document that described the common practices was published in November 1996. This was known as <a href=\"https://datatracker.ietf.org/doc/html/rfc1945\" class=\"external\" target=\"_blank\">RFC 1945</a> and defined HTTP/1.0.</p>"}},{"type":"prose","value":{"id":"http1.1_–_the_standardized_protocol","title":"HTTP/1.1 – The standardized protocol","isH3":false,"content":"<p>In the meantime, proper standardization was in progress. This happened in parallel to the diverse implementations of HTTP/1.0. The first standardized version of HTTP, HTTP/1.1, was published in early 1997, only a few months after HTTP/1.0.</p>\n<p>HTTP/1.1 clarified ambiguities and introduced numerous improvements:</p>\n<ul>\n<li>A connection could be reused, which saved time. It no longer needed to be opened multiple times to display the resources embedded in the single original document.</li>\n<li>Pipelining was added. This allowed a second request to be sent before the answer to the first one was fully transmitted. This lowered the latency of the communication.</li>\n<li>Chunked responses were also supported.</li>\n<li>Additional cache control mechanisms were introduced.</li>\n<li>Content negotiation, including language, encoding, and type, was introduced. A client and a server could now agree on which content to exchange.</li>\n<li>Thanks to the <a href=\"/en-US/docs/Web/HTTP/Headers/Host\"><code>Host</code></a> header, the ability to host different domains from the same IP address allowed server collocation.</li>\n</ul>\n<p>A typical flow of requests, all through one single connection, looked like this:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">http</span></div><pre class=\"brush: http notranslate\"><code>GET /en-US/docs/Glossary/CORS-safelisted_request_header HTTP/1.1\nHost: developer.mozilla.org\nUser-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:50.0) Gecko/20100101 Firefox/50.0\nAccept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8\nAccept-Language: en-US,en;q=0.5\nAccept-Encoding: gzip, deflate, br\nReferer: https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted_request_header\n\nHTTP/1.1 200 OK\nConnection: Keep-Alive\nContent-Encoding: gzip\nContent-Type: text/html; charset=utf-8\nDate: Wed, 20 Jul 2016 10:55:30 GMT\nEtag: \"547fa7e369ef56031dd3bff2ace9fc0832eb251a\"\nKeep-Alive: timeout=5, max=1000\nLast-Modified: Tue, 19 Jul 2016 00:59:33 GMT\nServer: Apache\nTransfer-Encoding: chunked\nVary: Cookie, Accept-Encoding\n\n(content)\n\nGET /static/img/header-background.png HTTP/1.1\nHost: developer.mozilla.org\nUser-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:50.0) Gecko/20100101 Firefox/50.0\nAccept: */*\nAccept-Language: en-US,en;q=0.5\nAccept-Encoding: gzip, deflate, br\nReferer: https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted_request_header\n\nHTTP/1.1 200 OK\nAge: 9578461\nCache-Control: public, max-age=315360000\nConnection: keep-alive\nContent-Length: 3077\nContent-Type: image/png\nDate: Thu, 31 Mar 2016 13:34:46 GMT\nLast-Modified: Wed, 21 Oct 2015 18:27:50 GMT\nServer: Apache\n\n(image content of 3077 bytes)\n</code></pre></div>\n<p>HTTP/1.1 was first published as <a href=\"https://datatracker.ietf.org/doc/html/rfc2068\" class=\"external\" target=\"_blank\">RFC 2068</a> in January 1997.</p>"}},{"type":"prose","value":{"id":"more_than_two_decades_of_development","title":"More than two decades of development","isH3":false,"content":"<p>The extensibility of HTTP made it easy to create new headers and methods. Even though the HTTP/1.1 protocol was refined over two revisions, <a href=\"https://datatracker.ietf.org/doc/html/rfc2616\" class=\"external\" target=\"_blank\">RFC 2616</a> published in June 1999 and <a href=\"https://datatracker.ietf.org/doc/html/rfc7230\" class=\"external\" target=\"_blank\">RFC 7230</a>-<a href=\"https://datatracker.ietf.org/doc/html/rfc7235\" class=\"external\" target=\"_blank\">RFC 7235</a> published in June 2014 before the release of HTTP/2, it was extremely stable for more than 15 years. HTTP/1.1 was updated again in 2022 with <a href=\"https://datatracker.ietf.org/doc/html/rfc9110\" class=\"external\" target=\"_blank\">RFC 9110</a>. Not only was HTTP/1.1 updated, but all of HTTP was revised and is now split into the following documents: semantics (<a href=\"https://datatracker.ietf.org/doc/html/rfc9110\" class=\"external\" target=\"_blank\">RFC 9110</a>), caching (<a href=\"https://datatracker.ietf.org/doc/html/rfc9111\" class=\"external\" target=\"_blank\">RFC 9111</a>) applying to all HTTP versions, and HTTP/1.1 (<a href=\"https://datatracker.ietf.org/doc/html/rfc9112\" class=\"external\" target=\"_blank\">RFC 9112</a>), HTTP/2 (<a href=\"https://datatracker.ietf.org/doc/html/rfc9113\" class=\"external\" target=\"_blank\">RFC 9113</a>), and HTTP/3 (<a href=\"https://datatracker.ietf.org/doc/html/rfc9114\" class=\"external\" target=\"_blank\">RFC 9114</a>). In addition, the specification finally achieved the status of Internet Standard (STD 97), whereas before it was always a proposed/draft standard.</p>"}},{"type":"prose","value":{"id":"using_http_for_secure_transmissions","title":"Using HTTP for secure transmissions","isH3":true,"content":"<p>The largest change to HTTP was made at the end of 1994. Instead of sending HTTP over a basic TCP/IP stack, the computer-services company Netscape Communications created an additional encrypted transmission layer on top of it: SSL. SSL 1.0 was never released to the public, but SSL 2.0 and its successor SSL 3.0 allowed for the creation of e-commerce websites. To do this, they encrypted and guaranteed the authenticity of the messages exchanged between the server and client. SSL was eventually standardized and became TLS.</p>\n<p>During the same time period, it became clear that an encrypted transport layer was needed. The web was no longer a mostly academic network, and instead became a jungle where advertisers, random individuals, and criminals competed for as much private data as possible. As the applications built over HTTP became more powerful and required access to private information like address books, email, and user location, TLS became necessary outside the e-commerce use case.</p>"}},{"type":"prose","value":{"id":"using_http_for_complex_applications","title":"Using HTTP for complex applications","isH3":true,"content":"<p>Tim Berners-Lee didn't originally envision HTTP as a read-only medium. He wanted to create a web where people could add and move documents remotely—a kind of distributed file system. Around 1996, HTTP was extended to allow authoring, and a standard called WebDAV was created. It grew to include specific applications like CardDAV for handling address book entries and CalDAV for dealing with calendars. But all these *DAV extensions had a flaw: they were only usable when implemented by the servers.</p>\n<p>In 2000, a new pattern for using HTTP was designed: <a href=\"/en-US/docs/Glossary/REST\">representational state transfer</a> (or REST). The API wasn't based on the new HTTP methods, but instead relied on access to specific URIs with basic HTTP/1.1 methods. This allowed any web application to let an API retrieve and modify its data without having to update the browsers or the servers. All necessary information was embedded in the files that the websites served through standard HTTP/1.1. The drawback of the REST model was that each website defined its own nonstandard RESTful API and had total control of it. This differed from the *DAV extensions where clients and servers were interoperable. RESTful APIs became very common in the 2010s.</p>\n<p>Since 2005, more APIs have become available to web pages. Several of these APIs create extensions to the HTTP protocol for specific purposes:</p>\n<ul>\n<li><a href=\"/en-US/docs/Web/API/Server-sent_events\">Server-sent events</a>, where the server can push occasional messages to the browser.</li>\n<li><a href=\"/en-US/docs/Web/API/WebSockets_API\">WebSocket</a>, a new protocol that can be set up by upgrading an existing HTTP connection.</li>\n</ul>"}},{"type":"prose","value":{"id":"relaxing_the_security-model_of_the_web","title":"Relaxing the security-model of the web","isH3":true,"content":"<p>HTTP is independent of the web security model, known as the <a href=\"/en-US/docs/Web/Security/Same-origin_policy\">same-origin policy</a>. In fact, the current web security model was developed after the creation of HTTP! Over the years, it proved useful to lift some restrictions of this policy under certain constraints. The server transmitted how much and when to lift such restrictions to the client using a new set of HTTP headers. These were defined in specifications like <a href=\"/en-US/docs/Glossary/CORS\">Cross-Origin Resource Sharing</a> (CORS) and the <a href=\"/en-US/docs/Web/HTTP/CSP\">Content Security Policy</a> (CSP).</p>\n<p>In addition to these large extensions, many other headers were added, sometimes only experimentally. Notable headers are the Do Not Track (<a href=\"/en-US/docs/Web/HTTP/Headers/DNT\"><code>DNT</code></a>) header to control privacy, <a href=\"/en-US/docs/Web/HTTP/Headers/X-Frame-Options\"><code>X-Frame-Options</code></a>, and <a href=\"/en-US/docs/Web/HTTP/Headers/Upgrade-Insecure-Requests\"><code>Upgrade-Insecure-Requests</code></a> but many more exist.</p>"}},{"type":"prose","value":{"id":"http2_–_a_protocol_for_greater_performance","title":"HTTP/2 – A protocol for greater performance","isH3":false,"content":"<p>Over the years, web pages became more complex. Some of them were even applications in their own right. More visual media was displayed and the volume and size of scripts adding interactivity also increased. Much more data was transmitted over significantly more HTTP requests and this created more complexity and overhead for HTTP/1.1 connections. To account for this, Google implemented an experimental protocol SPDY in the early 2010s. This alternative way of exchanging data between client and server amassed interest from developers working on both browsers and servers. SPDY defined an increase in responsiveness and solved the problem of duplicate data transmission, serving as the foundation for the HTTP/2 protocol.</p>\n<p>The HTTP/2 protocol differs from HTTP/1.1 in a few ways:</p>\n<ul>\n<li>It's a binary protocol rather than a text protocol. It can't be read and created manually. Despite this hurdle, it allows for the implementation of improved optimization techniques.</li>\n<li>It's a multiplexed protocol. Parallel requests can be made over the same connection, removing the constraints of the HTTP/1.x protocol.</li>\n<li>It compresses headers. As these are often similar among a set of requests, this removes the duplication and overhead of data transmitted.</li>\n</ul>\n<p>Officially standardized in May 2015, HTTP/2 use peaked in January 2022 at 46.9% of all websites (see <a href=\"https://w3techs.com/technologies/details/ce-http2\" class=\"external\" target=\"_blank\">these stats</a>). High-traffic websites showed the most rapid adoption in an effort to save on data transfer overhead and subsequent budgets.</p>\n<p>This rapid adoption was likely because HTTP/2 didn't require changes to websites and applications. To use it, only an up-to-date server that communicated with a recent browser was necessary. Only a limited set of groups was needed to trigger adoption, and as legacy browser and server versions were renewed, usage was naturally increased, without significant work for web developers.</p>"}},{"type":"prose","value":{"id":"post-http2_evolution","title":"Post-HTTP/2 evolution","isH3":false,"content":"<p>HTTP's extensibility is still being used to add new features. Notably, we can cite new extensions of the HTTP protocol that appeared in 2016:</p>\n<ul>\n<li>Support for <a href=\"/en-US/docs/Web/HTTP/Headers/Alt-Svc\"><code>Alt-Svc</code></a> allowed the dissociation of the identification and the location of a given resource. This meant a smarter <a href=\"/en-US/docs/Glossary/CDN\">CDN</a> caching mechanism.</li>\n<li>The introduction of <a href=\"/en-US/docs/Web/HTTP/Client_hints\">client hints</a> allowed the browser or client to proactively communicate information about its requirements and hardware constraints to the server.</li>\n<li>The introduction of security-related prefixes in the <a href=\"/en-US/docs/Web/HTTP/Headers/Cookie\"><code>Cookie</code></a> header helped guarantee that secure cookies couldn't be altered.</li>\n</ul>"}},{"type":"prose","value":{"id":"http3_-_http_over_quic","title":"HTTP/3 - HTTP over QUIC","isH3":false,"content":"<p>The next major version of HTTP, HTTP/3 has the same semantics as earlier versions of HTTP but uses <a href=\"/en-US/docs/Glossary/QUIC\">QUIC</a> instead of <a href=\"/en-US/docs/Glossary/TCP\">TCP</a> for the transport layer portion. By October 2022, <a href=\"https://w3techs.com/technologies/details/ce-http3\" class=\"external\" target=\"_blank\">26% of all websites were using HTTP/3</a>.</p>\n<p>QUIC is designed to provide much lower latency for HTTP connections. Like HTTP/2, it is a multiplexed protocol, but HTTP/2 runs over a single TCP connection, so packet loss detection and retransmission handled at the TCP layer can block all streams. QUIC runs multiple streams over <a href=\"/en-US/docs/Glossary/UDP\">UDP</a> and implements packet loss detection and retransmission independently for each stream, so that if an error occurs, only the stream with data in that packet is blocked.</p>\n<p>Defined in <a href=\"https://datatracker.ietf.org/doc/html/rfc9114\" class=\"external\" target=\"_blank\">RFC 9114</a>, <a href=\"https://caniuse.com/http3\" class=\"external\" target=\"_blank\">HTTP/3 is supported by most major browsers</a> including Chromium (and its variants such as Chrome and Edge) and Firefox.</p>"}},{"type":"prose","value":{"id":"see_also","title":"See also","isH3":false,"content":"<ul>\n<li><a href=\"/en-US/docs/Web/HTTP/Connection_management_in_HTTP_1.x\">Connection management in HTTP/1.x</a></li>\n<li><a href=\"/en-US/docs/Web/HTTP/Protocol_upgrade_mechanism\">Protocol upgrade mechanism</a></li>\n<li><a href=\"/en-US/docs/Web/HTTP/Resources_and_specifications\">HTTP resources and specifications</a></li>\n<li>Glossary terms:\n<ul>\n<li><a href=\"/en-US/docs/Glossary/HTTP\">HTTP</a></li>\n<li><a href=\"/en-US/docs/Glossary/HTTP_2\">HTTP/2</a></li>\n<li><a href=\"/en-US/docs/Glossary/QUIC\">QUIC</a></li>\n<li><a href=\"/en-US/docs/Glossary/Round_Trip_Time\">Round Trip Time (RTT)</a></li>\n<li><a href=\"/en-US/docs/Glossary/TCP_slow_start\">TCP slow start</a></li>\n<li><a href=\"/en-US/docs/Glossary/TCP\">Transmission Control Protocol (TCP)</a></li>\n</ul>\n</li>\n</ul>"}}],"isActive":true,"isMarkdown":true,"isTranslated":false,"locale":"en-US","mdn_url":"/en-US/docs/Web/HTTP/Evolution_of_HTTP","modified":"2024-12-16T09:54:57.000Z","native":"English (US)","noIndexing":false,"other_translations":[{"locale":"de","title":"Evolution von HTTP","native":"Deutsch"},{"locale":"es","title":"Evolución del protocolo HTTP","native":"Español"},{"locale":"fr","title":"L'évolution du protocole HTTP","native":"Français"},{"locale":"ja","title":"HTTP の進化","native":"日本語"},{"locale":"ko","title":"HTTP의 진화","native":"한국어"},{"locale":"zh-CN","title":"HTTP 的发展","native":"中文 (简体)"}],"pageTitle":"Evolution of HTTP - HTTP | MDN","parents":[{"uri":"/en-US/docs/Web","title":"References"},{"uri":"/en-US/docs/Web/HTTP","title":"HTTP"},{"uri":"/en-US/docs/Web/HTTP/Evolution_of_HTTP","title":"Evolution of HTTP"}],"popularity":null,"short_title":"Evolution of HTTP","sidebarHTML":"<ol><li class=\"section\"><a href=\"/en-US/docs/Web/HTTP\">HTTP</a></li><li class=\"section\">Guides</li><li><a href=\"/en-US/docs/Web/HTTP/Overview\">An overview of HTTP</a></li><li><a href=\"/en-US/docs/Web/HTTP/Session\">A typical HTTP session</a></li><li><a href=\"/en-US/docs/Web/HTTP/Messages\">HTTP messages</a></li><li><a href=\"/en-US/docs/Web/HTTP/MIME_types\">MIME types (IANA media types)</a></li><li><a href=\"/en-US/docs/Web/HTTP/Compression\">Compression in HTTP</a></li><li><a href=\"/en-US/docs/Web/HTTP/Caching\">HTTP caching</a></li><li><a href=\"/en-US/docs/Web/HTTP/Authentication\">HTTP authentication</a></li><li><a href=\"/en-US/docs/Web/HTTP/Cookies\">Using HTTP cookies</a></li><li><a href=\"/en-US/docs/Web/HTTP/Redirections\">Redirections in HTTP</a></li><li><a href=\"/en-US/docs/Web/HTTP/Conditional_requests\">HTTP conditional requests</a></li><li><a href=\"/en-US/docs/Web/HTTP/Range_requests\">HTTP range requests</a></li><li><a href=\"/en-US/docs/Web/HTTP/Content_negotiation\">Content negotiation</a></li><li><a href=\"/en-US/docs/Web/HTTP/Connection_management_in_HTTP_1.x\">Connection management in HTTP/1.x</a></li><li><em><a href=\"/en-US/docs/Web/HTTP/Evolution_of_HTTP\" aria-current=\"page\">Evolution of HTTP</a></em></li><li><a href=\"/en-US/docs/Web/HTTP/Protocol_upgrade_mechanism\">Protocol upgrade mechanism</a></li><li><a href=\"/en-US/docs/Web/HTTP/Proxy_servers_and_tunneling\">Proxy servers and tunneling</a></li><li><a href=\"/en-US/docs/Web/HTTP/Client_hints\">HTTP Client hints</a></li><li class=\"toggle\"><details><summary>Security and privacy</summary><ol><li><a href=\"/en-US/docs/Web/Security/Practical_implementation_guides\">Practical security implementation guides</a></li><li><a href=\"/en-US/observatory\">HTTP Observatory</a></li><li><a href=\"/en-US/docs/Web/HTTP/Permissions_Policy\">Permissions Policy</a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/CSP\">Content Security Policy (CSP)</a></li><li><a href=\"/en-US/docs/Web/HTTP/CORS\">Cross-Origin Resource Sharing (CORS)</a></li><li><a href=\"/en-US/docs/Web/HTTP/Cross-Origin_Resource_Policy\">Cross-Origin Resource Policy (CORP)</a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers\">Headers</a></li></ol></details></li><li class=\"section\">References</li><li class=\"toggle\"><details><summary>HTTP headers</summary><ol><li><a href=\"/en-US/docs/Web/HTTP/Headers/Accept\"><code>Accept</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Accept-CH\"><code>Accept-CH</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Accept-Encoding\"><code>Accept-Encoding</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Accept-Language\"><code>Accept-Language</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Accept-Patch\"><code>Accept-Patch</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Accept-Post\"><code>Accept-Post</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Accept-Ranges\"><code>Accept-Ranges</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Credentials\"><code>Access-Control-Allow-Credentials</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Headers\"><code>Access-Control-Allow-Headers</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Methods\"><code>Access-Control-Allow-Methods</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Origin\"><code>Access-Control-Allow-Origin</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Access-Control-Expose-Headers\"><code>Access-Control-Expose-Headers</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Access-Control-Max-Age\"><code>Access-Control-Max-Age</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Access-Control-Request-Headers\"><code>Access-Control-Request-Headers</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Access-Control-Request-Method\"><code>Access-Control-Request-Method</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Age\"><code>Age</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Allow\"><code>Allow</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Alt-Svc\"><code>Alt-Svc</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Alt-Used\"><code>Alt-Used</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Attribution-Reporting-Eligible\"><code>Attribution-Reporting-Eligible</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Attribution-Reporting-Register-Source\"><code>Attribution-Reporting-Register-Source</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Attribution-Reporting-Register-Trigger\"><code>Attribution-Reporting-Register-Trigger</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Authorization\"><code>Authorization</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Cache-Control\"><code>Cache-Control</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Clear-Site-Data\"><code>Clear-Site-Data</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Connection\"><code>Connection</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Digest\"><code>Content-Digest</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Disposition\"><code>Content-Disposition</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-DPR\"><code>Content-DPR</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Encoding\"><code>Content-Encoding</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Language\"><code>Content-Language</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Length\"><code>Content-Length</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Location\"><code>Content-Location</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Range\"><code>Content-Range</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy\"><code>Content-Security-Policy</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy-Report-Only\"><code>Content-Security-Policy-Report-Only</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Type\"><code>Content-Type</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Cookie\"><code>Cookie</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Critical-CH\"><code>Critical-CH</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Cross-Origin-Embedder-Policy\"><code>Cross-Origin-Embedder-Policy</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Cross-Origin-Opener-Policy\"><code>Cross-Origin-Opener-Policy</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Cross-Origin-Resource-Policy\"><code>Cross-Origin-Resource-Policy</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Date\"><code>Date</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Device-Memory\"><code>Device-Memory</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/DNT\"><code>DNT</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Downlink\"><code>Downlink</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/DPR\"><code>DPR</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Early-Data\"><code>Early-Data</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/ECT\"><code>ECT</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/ETag\"><code>ETag</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Expect\"><code>Expect</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Expect-CT\"><code>Expect-CT</code></a><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Expires\"><code>Expires</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Forwarded\"><code>Forwarded</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/From\"><code>From</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Host\"><code>Host</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/If-Match\"><code>If-Match</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/If-Modified-Since\"><code>If-Modified-Since</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/If-None-Match\"><code>If-None-Match</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/If-Range\"><code>If-Range</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/If-Unmodified-Since\"><code>If-Unmodified-Since</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Keep-Alive\"><code>Keep-Alive</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Last-Modified\"><code>Last-Modified</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Link\"><code>Link</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Location\"><code>Location</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Max-Forwards\"><code>Max-Forwards</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/NEL\"><code>NEL</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/No-Vary-Search\"><code>No-Vary-Search</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Observe-Browsing-Topics\"><code>Observe-Browsing-Topics</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Origin\"><code>Origin</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Origin-Agent-Cluster\"><code>Origin-Agent-Cluster</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy\"><code>Permissions-Policy</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Pragma\"><code>Pragma</code></a><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Priority\"><code>Priority</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Proxy-Authenticate\"><code>Proxy-Authenticate</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Proxy-Authorization\"><code>Proxy-Authorization</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Range\"><code>Range</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Referer\"><code>Referer</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Referrer-Policy\"><code>Referrer-Policy</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Refresh\"><code>Refresh</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Report-To\"><code>Report-To</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Reporting-Endpoints\"><code>Reporting-Endpoints</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Repr-Digest\"><code>Repr-Digest</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Retry-After\"><code>Retry-After</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/RTT\"><code>RTT</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Save-Data\"><code>Save-Data</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-Browsing-Topics\"><code>Sec-Browsing-Topics</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-CH-Prefers-Color-Scheme\"><code>Sec-CH-Prefers-Color-Scheme</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-CH-Prefers-Reduced-Motion\"><code>Sec-CH-Prefers-Reduced-Motion</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-CH-Prefers-Reduced-Transparency\"><code>Sec-CH-Prefers-Reduced-Transparency</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-CH-UA\"><code>Sec-CH-UA</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Arch\"><code>Sec-CH-UA-Arch</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Bitness\"><code>Sec-CH-UA-Bitness</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Full-Version\"><code>Sec-CH-UA-Full-Version</code></a><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Full-Version-List\"><code>Sec-CH-UA-Full-Version-List</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Mobile\"><code>Sec-CH-UA-Mobile</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Model\"><code>Sec-CH-UA-Model</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Platform\"><code>Sec-CH-UA-Platform</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Platform-Version\"><code>Sec-CH-UA-Platform-Version</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-Fetch-Dest\"><code>Sec-Fetch-Dest</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-Fetch-Mode\"><code>Sec-Fetch-Mode</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-Fetch-Site\"><code>Sec-Fetch-Site</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-Fetch-User\"><code>Sec-Fetch-User</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-GPC\"><code>Sec-GPC</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-Purpose\"><code>Sec-Purpose</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-WebSocket-Accept\"><code>Sec-WebSocket-Accept</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-WebSocket-Extensions\"><code>Sec-WebSocket-Extensions</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-WebSocket-Key\"><code>Sec-WebSocket-Key</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-WebSocket-Protocol\"><code>Sec-WebSocket-Protocol</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Sec-WebSocket-Version\"><code>Sec-WebSocket-Version</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Server\"><code>Server</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Server-Timing\"><code>Server-Timing</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Service-Worker\"><code>Service-Worker</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Service-Worker-Allowed\"><code>Service-Worker-Allowed</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Service-Worker-Navigation-Preload\"><code>Service-Worker-Navigation-Preload</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Set-Cookie\"><code>Set-Cookie</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Set-Login\"><code>Set-Login</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/SourceMap\"><code>SourceMap</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Speculation-Rules\"><code>Speculation-Rules</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Strict-Transport-Security\"><code>Strict-Transport-Security</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Supports-Loading-Mode\"><code>Supports-Loading-Mode</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/TE\"><code>TE</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Timing-Allow-Origin\"><code>Timing-Allow-Origin</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Tk\"><code>Tk</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Trailer\"><code>Trailer</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Transfer-Encoding\"><code>Transfer-Encoding</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Upgrade\"><code>Upgrade</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Upgrade-Insecure-Requests\"><code>Upgrade-Insecure-Requests</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/User-Agent\"><code>User-Agent</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Vary\"><code>Vary</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Via\"><code>Via</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Viewport-Width\"><code>Viewport-Width</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Want-Content-Digest\"><code>Want-Content-Digest</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Want-Repr-Digest\"><code>Want-Repr-Digest</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Warning\"><code>Warning</code></a><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Width\"><code>Width</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/WWW-Authenticate\"><code>WWW-Authenticate</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/X-Content-Type-Options\"><code>X-Content-Type-Options</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/X-DNS-Prefetch-Control\"><code>X-DNS-Prefetch-Control</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/X-Forwarded-For\"><code>X-Forwarded-For</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/X-Forwarded-Host\"><code>X-Forwarded-Host</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/X-Forwarded-Proto\"><code>X-Forwarded-Proto</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/X-Frame-Options\"><code>X-Frame-Options</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/X-Permitted-Cross-Domain-Policies\"><code>X-Permitted-Cross-Domain-Policies</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/X-Powered-By\"><code>X-Powered-By</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/X-Robots-Tag\"><code>X-Robots-Tag</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/X-XSS-Protection\"><code>X-XSS-Protection</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li></ol></details></li><li class=\"toggle\"><details><summary>HTTP request methods</summary><ol><li><a href=\"/en-US/docs/Web/HTTP/Methods/CONNECT\"><code>CONNECT</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Methods/DELETE\"><code>DELETE</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Methods/GET\"><code>GET</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Methods/HEAD\"><code>HEAD</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Methods/OPTIONS\"><code>OPTIONS</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Methods/PATCH\"><code>PATCH</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Methods/POST\"><code>POST</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Methods/PUT\"><code>PUT</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Methods/TRACE\"><code>TRACE</code></a></li></ol></details></li><li class=\"toggle\"><details><summary>HTTP response status codes</summary><ol><li><a href=\"/en-US/docs/Web/HTTP/Status/100\"><code>100 Continue</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/101\"><code>101 Switching Protocols</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/102\"><code>102 Processing</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/103\"><code>103 Early Hints</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/200\"><code>200 OK</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/201\"><code>201 Created</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/202\"><code>202 Accepted</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/203\"><code>203 Non-Authoritative Information</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/204\"><code>204 No Content</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/205\"><code>205 Reset Content</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/206\"><code>206 Partial Content</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/207\"><code>207 Multi-Status</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/208\"><code>208 Already Reported</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/226\"><code>226 IM Used</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/300\"><code>300 Multiple Choices</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/301\"><code>301 Moved Permanently</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/302\"><code>302 Found</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/303\"><code>303 See Other</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/304\"><code>304 Not Modified</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/307\"><code>307 Temporary Redirect</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/308\"><code>308 Permanent Redirect</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/400\"><code>400 Bad Request</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/401\"><code>401 Unauthorized</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/402\"><code>402 Payment Required</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/403\"><code>403 Forbidden</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/404\"><code>404 Not Found</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/405\"><code>405 Method Not Allowed</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/406\"><code>406 Not Acceptable</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/407\"><code>407 Proxy Authentication Required</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/408\"><code>408 Request Timeout</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/409\"><code>409 Conflict</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/410\"><code>410 Gone</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/411\"><code>411 Length Required</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/412\"><code>412 Precondition Failed</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/413\"><code>413 Content Too Large</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/414\"><code>414 URI Too Long</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/415\"><code>415 Unsupported Media Type</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/416\"><code>416 Range Not Satisfiable</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/417\"><code>417 Expectation Failed</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/418\"><code>418 I'm a teapot</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/421\"><code>421 Misdirected Request</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/422\"><code>422 Unprocessable Content</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/423\"><code>423 Locked</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/424\"><code>424 Failed Dependency</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/425\"><code>425 Too Early</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/426\"><code>426 Upgrade Required</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/428\"><code>428 Precondition Required</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/429\"><code>429 Too Many Requests</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/431\"><code>431 Request Header Fields Too Large</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/451\"><code>451 Unavailable For Legal Reasons</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/500\"><code>500 Internal Server Error</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/501\"><code>501 Not Implemented</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/502\"><code>502 Bad Gateway</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/503\"><code>503 Service Unavailable</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/504\"><code>504 Gateway Timeout</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/505\"><code>505 HTTP Version Not Supported</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/506\"><code>506 Variant Also Negotiates</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/507\"><code>507 Insufficient Storage</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/508\"><code>508 Loop Detected</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/510\"><code>510 Not Extended</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Status/511\"><code>511 Network Authentication Required</code></a></li></ol></details></li><li class=\"toggle\"><details><summary>CSP directives</summary><ol><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/base-uri\"><code>CSP: base-uri</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/block-all-mixed-content\"><code>CSP: block-all-mixed-content</code></a><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/child-src\"><code>CSP: child-src</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/connect-src\"><code>CSP: connect-src</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/default-src\"><code>CSP: default-src</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/fenced-frame-src\"><code>CSP: fenced-frame-src</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/font-src\"><code>CSP: font-src</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/form-action\"><code>CSP: form-action</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/frame-ancestors\"><code>CSP: frame-ancestors</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/frame-src\"><code>CSP: frame-src</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/img-src\"><code>CSP: img-src</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/manifest-src\"><code>CSP: manifest-src</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/media-src\"><code>CSP: media-src</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/object-src\"><code>CSP: object-src</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/prefetch-src\"><code>CSP: prefetch-src</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/report-to\"><code>CSP: report-to</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/report-uri\"><code>CSP: report-uri</code></a><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n<span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/require-trusted-types-for\"><code>CSP: require-trusted-types-for</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/sandbox\"><code>CSP: sandbox</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src\"><code>CSP: script-src</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src-attr\"><code>CSP: script-src-attr</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src-elem\"><code>CSP: script-src-elem</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/style-src\"><code>CSP: style-src</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/style-src-attr\"><code>CSP: style-src-attr</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/style-src-elem\"><code>CSP: style-src-elem</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/trusted-types\"><code>CSP: trusted-types</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/upgrade-insecure-requests\"><code>CSP: upgrade-insecure-requests</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/worker-src\"><code>CSP: worker-src</code></a></li></ol></details></li><li class=\"toggle\"><details><summary>CORS errors</summary><ol><li><a href=\"/en-US/docs/Web/HTTP/CORS/Errors/CORSDisabled\"><code>Reason: CORS disabled</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/CORS/Errors/CORSAllowOriginNotMatchingOrigin\"><code>Reason: CORS header 'Access-Control-Allow-Origin' does not match 'xyz'</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/CORS/Errors/CORSMissingAllowOrigin\"><code>Reason: CORS header 'Access-Control-Allow-Origin' missing</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/CORS/Errors/CORSOriginHeaderNotAdded\"><code>Reason: CORS header 'Origin' cannot be added</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/CORS/Errors/CORSPreflightDidNotSucceed\"><code>Reason: CORS preflight channel did not succeed</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/CORS/Errors/CORSDidNotSucceed\"><code>Reason: CORS request did not succeed</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/CORS/Errors/CORSExternalRedirectNotAllowed\"><code>Reason: CORS request external redirect not allowed</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/CORS/Errors/CORSRequestNotHttp\"><code>Reason: CORS request not HTTP</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/CORS/Errors/CORSNotSupportingCredentials\"><code>Reason: Credential is not supported if the CORS header 'Access-Control-Allow-Origin' is '*'</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/CORS/Errors/CORSMethodNotFound\"><code>Reason: Did not find method in CORS header 'Access-Control-Allow-Methods'</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/CORS/Errors/CORSMIssingAllowCredentials\"><code>Reason: expected 'true' in CORS header 'Access-Control-Allow-Credentials'</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/CORS/Errors/CORSInvalidAllowHeader\"><code>Reason: invalid token 'xyz' in CORS header 'Access-Control-Allow-Headers'</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/CORS/Errors/CORSInvalidAllowMethod\"><code>Reason: invalid token 'xyz' in CORS header 'Access-Control-Allow-Methods'</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/CORS/Errors/CORSMissingAllowHeaderFromPreflight\"><code>Reason: missing token 'xyz' in CORS header 'Access-Control-Allow-Headers' from CORS preflight channel</code></a></li><li><a href=\"/en-US/docs/Web/HTTP/CORS/Errors/CORSMultipleAllowOriginNotAllowed\"><code>Reason: Multiple CORS header 'Access-Control-Allow-Origin' not allowed</code></a></li></ol></details></li><li class=\"toggle\"><details><summary>Permissions-Policy directives</summary><ol><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/accelerometer\"><code>Permissions-Policy: accelerometer</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/ambient-light-sensor\"><code>Permissions-Policy: ambient-light-sensor</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/attribution-reporting\"><code>Permissions-Policy: attribution-reporting</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/autoplay\"><code>Permissions-Policy: autoplay</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/bluetooth\"><code>Permissions-Policy: bluetooth</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/browsing-topics\"><code>Permissions-Policy: browsing-topics</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n<span class=\"visually-hidden\">Non-standard</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/camera\"><code>Permissions-Policy: camera</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/compute-pressure\"><code>Permissions-Policy: compute-pressure</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/cross-origin-isolated\"><code>Permissions-Policy: cross-origin-isolated</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/display-capture\"><code>Permissions-Policy: display-capture</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/document-domain\"><code>Permissions-Policy: document-domain</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/encrypted-media\"><code>Permissions-Policy: encrypted-media</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/fullscreen\"><code>Permissions-Policy: fullscreen</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/gamepad\"><code>Permissions-Policy: gamepad</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/geolocation\"><code>Permissions-Policy: geolocation</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/gyroscope\"><code>Permissions-Policy: gyroscope</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/hid\"><code>Permissions-Policy: hid</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/identity-credentials-get\"><code>Permissions-Policy: identity-credentials-get</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/idle-detection\"><code>Permissions-Policy: idle-detection</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/local-fonts\"><code>Permissions-Policy: local-fonts</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/magnetometer\"><code>Permissions-Policy: magnetometer</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/microphone\"><code>Permissions-Policy: microphone</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/midi\"><code>Permissions-Policy: midi</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/otp-credentials\"><code>Permissions-Policy: otp-credentials</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/payment\"><code>Permissions-Policy: payment</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/picture-in-picture\"><code>Permissions-Policy: picture-in-picture</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/publickey-credentials-create\"><code>Permissions-Policy: publickey-credentials-create</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/publickey-credentials-get\"><code>Permissions-Policy: publickey-credentials-get</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/screen-wake-lock\"><code>Permissions-Policy: screen-wake-lock</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/serial\"><code>Permissions-Policy: serial</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/speaker-selection\"><code>Permissions-Policy: speaker-selection</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/storage-access\"><code>Permissions-Policy: storage-access</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/usb\"><code>Permissions-Policy: usb</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/web-share\"><code>Permissions-Policy: web-share</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/window-management\"><code>Permissions-Policy: window-management</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/HTTP/Headers/Permissions-Policy/xr-spatial-tracking\"><code>Permissions-Policy: xr-spatial-tracking</code></a><abbr class=\"icon icon-experimental\" title=\"Experimental. Expect behavior to change in the future.\">\n<span class=\"visually-hidden\">Experimental</span>\n</abbr></li></ol></details></li><li><a href=\"/en-US/docs/Web/HTTP/Resources_and_specifications\">HTTP resources and specifications</a></li></ol>","source":{"folder":"en-us/web/http/evolution_of_http","github_url":"https://github.com/mdn/content/blob/main/files/en-us/web/http/evolution_of_http/index.md","last_commit_url":"https://github.com/mdn/content/commit/ab1bf2c5955c1bfa4d96d779f701ab22f3870d43","filename":"index.md"},"summary":"HTTP (HyperText Transfer Protocol) is the underlying protocol of the World Wide Web. Developed by Tim Berners-Lee and his team between 1989-1991, HTTP has gone through many changes that have helped maintain its simplicity while shaping its flexibility. Keep reading to learn how HTTP evolved from a protocol designed to exchange files in a semitrusted laboratory environment into a modern internet maze that carries images and videos in high resolution and 3D.","title":"Evolution of HTTP","toc":[{"text":"Invention of the World Wide Web","id":"invention_of_the_world_wide_web"},{"text":"HTTP/0.9 – The one-line protocol","id":"http0.9_–_the_one-line_protocol"},{"text":"HTTP/1.0 – Building extensibility","id":"http1.0_–_building_extensibility"},{"text":"HTTP/1.1 – The standardized protocol","id":"http1.1_–_the_standardized_protocol"},{"text":"More than two decades of development","id":"more_than_two_decades_of_development"},{"text":"HTTP/2 – A protocol for greater performance","id":"http2_–_a_protocol_for_greater_performance"},{"text":"Post-HTTP/2 evolution","id":"post-http2_evolution"},{"text":"HTTP/3 - HTTP over QUIC","id":"http3_-_http_over_quic"},{"text":"See also","id":"see_also"}],"pageType":"guide"}}</script></body></html>