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While marking up the form itself is easy, checking whether each field has a valid and coherent value is more difficult, and informing the user about the problem may become a headache. HTML5 introduced new mechanisms for forms: it added new semantic types for the <input> element and constraint validation to ease the work of checking the form content on the client side. Basic, usual constraints can be checked, without the need for JavaScript, by setting new attributes; more complex constraints can be tested using the Constraint Validation API."/><meta property="og:url" content="https://developer.mozilla.org/en-US/docs/Web/HTML/Constraint_validation"/><meta property="og:title" content="Constraint validation - HTML: HyperText Markup Language | MDN"/><meta property="og:type" content="website"/><meta property="og:locale" content="en_US"/><meta property="og:description" content="The creation of web forms has always been a complex task. While marking up the form itself is easy, checking whether each field has a valid and coherent value is more difficult, and informing the user about the problem may become a headache. HTML5 introduced new mechanisms for forms: it added new semantic types for the <input> element and constraint validation to ease the work of checking the form content on the client side. 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(Click to learn more.)"><span class="icon icon-question-mark "></span></a></div></form></li><li class=" "><a data-locale="de" href="/de/docs/Web/HTML/Constraint_validation" class="button submenu-item"><span>Deutsch</span><span title="Diese Übersetzung ist Teil eines Experiments."><span class="icon icon-experimental "></span></span></a></li><li class=" "><a data-locale="fr" href="/fr/docs/Web/HTML/Constraint_validation" class="button submenu-item"><span>Français</span></a></li><li class=" "><a data-locale="ja" href="/ja/docs/Web/HTML/Constraint_validation" class="button submenu-item"><span>日本語</span></a></li><li class=" "><a data-locale="ru" href="/ru/docs/Web/HTML/Constraint_validation" class="button submenu-item"><span>Русский</span></a></li><li class=" "><a data-locale="zh-CN" href="/zh-CN/docs/Web/HTML/Constraint_validation" class="button submenu-item"><span>中文 (简体)</span></a></li></ul></div></div></li></ul></div></div></div></div><div class="main-wrapper"><div class="sidebar-container"><aside id="sidebar-quicklinks" class="sidebar" data-macro="HTMLSidebar"><button type="button" class="button action backdrop" aria-label="Collapse sidebar"><span class="button-wrap"></span></button><nav aria-label="Related Topics" class="sidebar-inner"><header class="sidebar-actions"><section class="sidebar-filter-container"><div class="sidebar-filter "><label id="sidebar-filter-label" class="sidebar-filter-label" for="sidebar-filter-input"><span class="icon icon-filter"></span><span class="visually-hidden">Filter sidebar</span></label><input id="sidebar-filter-input" autoComplete="off" class="sidebar-filter-input-field false" type="text" placeholder="Filter" value=""/><button type="button" class="button action has-icon clear-sidebar-filter-button"><span class="button-wrap"><span class="icon icon-cancel "></span><span class="visually-hidden">Clear filter input</span></span></button></div></section></header><div class="sidebar-inner-nav"><div class="in-nav-toc"><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="#intrinsic_and_basic_constraints">Intrinsic and basic constraints</a></li><li class="document-toc-item "><a class="document-toc-link" href="#constraint_validation_process">Constraint validation process</a></li><li class="document-toc-item "><a class="document-toc-link" href="#complex_constraints_using_the_constraint_validation_api">Complex constraints using the Constraint Validation API</a></li><li class="document-toc-item "><a class="document-toc-link" href="#visual_styling_of_constraint_validation">Visual styling of constraint validation</a></li></ul></section></div></div><div class="sidebar-body"> <ol> <li class="section"><a href="/en-US/docs/Web/HTML">HTML</a></li> <li class="section"><a href="/en-US/docs/Learn/HTML">Tutorials</a></li> <li><a href="/en-US/docs/Learn/Getting_started_with_the_web/HTML_basics">HTML basics</a></li> <li class="toggle"> <details> <summary>Introduction to HTML</summary> <ol> <li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML">Introduction to HTML overview</a></li> <li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Getting_started">Getting started with HTML</a></li> <li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/The_head_metadata_in_HTML">What's in the head? Metadata in HTML</a></li> <li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/HTML_text_fundamentals">HTML text fundamentals</a></li> <li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Creating_hyperlinks">Creating hyperlinks</a></li> <li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Advanced_text_formatting">Advanced text formatting</a></li> <li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Document_and_website_structure">Document and website structure</a></li> <li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Debugging_HTML">Debugging HTML</a></li> <li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Marking_up_a_letter">Assessment: Marking up a letter</a></li> <li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Structuring_a_page_of_content">Assessment: Structuring a page of content</a></li> </ol> </details> </li> <li class="toggle"> <details> <summary>Multimedia and embedding</summary> <ol> <li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding">Multimedia and embedding overview</a></li> <li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Images_in_HTML">Images in HTML</a></li> <li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Video_and_audio_content">Video and audio content</a></li> <li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Other_embedding_technologies">From object to iframe — other embedding technologies</a></li> <li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Adding_vector_graphics_to_the_Web">Adding vector graphics to the web</a></li> <li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Responsive_images">Responsive images</a></li> <li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Mozilla_splash_page">Assessment: Mozilla splash page</a></li> </ol> </details> </li> <li class="toggle"> <details> <summary>HTML tables</summary> <ol> <li><a href="/en-US/docs/Learn/HTML/Tables">HTML tables overview</a></li> <li><a href="/en-US/docs/Learn/HTML/Tables/Basics">HTML table basics</a></li> <li><a href="/en-US/docs/Learn/HTML/Tables/Advanced">HTML table advanced features and accessibility</a></li> <li><a href="/en-US/docs/Learn/HTML/Tables/Structuring_planet_data">Assessment: Structuring planet data</a></li> </ol> </details> </li> <li class="section"><a href="/en-US/docs/Web/HTML/Reference">References</a></li> <li class="toggle"> <details> <summary>HTML elements</summary> <ol><li><a href="/en-US/docs/Web/HTML/Element/a"><code><a></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/abbr"><code><abbr></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/acronym"><code><acronym></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/HTML/Element/address"><code><address></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/area"><code><area></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/article"><code><article></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/aside"><code><aside></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/audio"><code><audio></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/b"><code><b></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/base"><code><base></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/bdi"><code><bdi></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/bdo"><code><bdo></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/big"><code><big></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/HTML/Element/blockquote"><code><blockquote></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/body"><code><body></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/br"><code><br></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/button"><code><button></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/canvas"><code><canvas></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/caption"><code><caption></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/center"><code><center></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/HTML/Element/cite"><code><cite></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/code"><code><code></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/col"><code><col></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/colgroup"><code><colgroup></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/data"><code><data></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/datalist"><code><datalist></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/dd"><code><dd></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/del"><code><del></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/details"><code><details></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/dfn"><code><dfn></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/dialog"><code><dialog></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/dir"><code><dir></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/HTML/Element/div"><code><div></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/dl"><code><dl></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/dt"><code><dt></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/em"><code><em></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/embed"><code><embed></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/fencedframe"><code><fencedframe></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/HTML/Element/fieldset"><code><fieldset></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/figcaption"><code><figcaption></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/figure"><code><figure></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/font"><code><font></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/HTML/Element/footer"><code><footer></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/form"><code><form></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/frame"><code><frame></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/HTML/Element/frameset"><code><frameset></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/HTML/Element/Heading_Elements"><code><h1></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/head"><code><head></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/header"><code><header></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/hgroup"><code><hgroup></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/hr"><code><hr></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/html"><code><html></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/i"><code><i></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/iframe"><code><iframe></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/img"><code><img></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input"><code><input></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/ins"><code><ins></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/kbd"><code><kbd></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/label"><code><label></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/legend"><code><legend></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/li"><code><li></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/link"><code><link></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/main"><code><main></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/map"><code><map></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/mark"><code><mark></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/marquee"><code><marquee></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/HTML/Element/menu"><code><menu></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/meta"><code><meta></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/meter"><code><meter></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/nav"><code><nav></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/nobr"><code><nobr></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/HTML/Element/noembed"><code><noembed></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/HTML/Element/noframes"><code><noframes></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/HTML/Element/noscript"><code><noscript></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/object"><code><object></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/ol"><code><ol></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/optgroup"><code><optgroup></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/option"><code><option></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/output"><code><output></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/p"><code><p></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/param"><code><param></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/HTML/Element/picture"><code><picture></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/plaintext"><code><plaintext></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/HTML/Element/portal"><code><portal></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/HTML/Element/pre"><code><pre></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/progress"><code><progress></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/q"><code><q></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/rb"><code><rb></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/HTML/Element/rp"><code><rp></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/rt"><code><rt></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/rtc"><code><rtc></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/HTML/Element/ruby"><code><ruby></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/s"><code><s></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/samp"><code><samp></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/script"><code><script></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/search"><code><search></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/section"><code><section></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/select"><code><select></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/slot"><code><slot></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/small"><code><small></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/source"><code><source></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/span"><code><span></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/strike"><code><strike></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/HTML/Element/strong"><code><strong></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/style"><code><style></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/sub"><code><sub></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/summary"><code><summary></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/sup"><code><sup></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/table"><code><table></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/tbody"><code><tbody></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/td"><code><td></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/template"><code><template></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/textarea"><code><textarea></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/tfoot"><code><tfoot></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/th"><code><th></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/thead"><code><thead></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/time"><code><time></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/title"><code><title></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/tr"><code><tr></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/track"><code><track></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/tt"><code><tt></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/HTML/Element/u"><code><u></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/ul"><code><ul></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/var"><code><var></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/video"><code><video></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/wbr"><code><wbr></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/xmp"><code><xmp></code></a><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>Global attributes</summary> <ol><li><a href="/en-US/docs/Web/HTML/Global_attributes/accesskey"><code>accesskey</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/anchor"><code>anchor</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/HTML/Global_attributes/autocapitalize"><code>autocapitalize</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/autocorrect"><code>autocorrect</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/HTML/Global_attributes/autofocus"><code>autofocus</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/class"><code>class</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/contenteditable"><code>contenteditable</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/data-*"><code>data-*</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/dir"><code>dir</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/draggable"><code>draggable</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/enterkeyhint"><code>enterkeyhint</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/exportparts"><code>exportparts</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/hidden"><code>hidden</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/id"><code>id</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/inert"><code>inert</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/inputmode"><code>inputmode</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/is"><code>is</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/itemid"><code>itemid</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/itemprop"><code>itemprop</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/itemref"><code>itemref</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/itemscope"><code>itemscope</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/itemtype"><code>itemtype</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/lang"><code>lang</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/nonce"><code>nonce</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/part"><code>part</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/popover"><code>popover</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/slot"><code>slot</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/spellcheck"><code>spellcheck</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/style"><code>style</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/tabindex"><code>tabindex</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/title"><code>title</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/translate"><code>translate</code></a></li><li><a href="/en-US/docs/Web/HTML/Global_attributes/virtualkeyboardpolicy"><code>virtualkeyboardpolicy</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/HTML/Global_attributes/writingsuggestions"><code>writingsuggestions</code></a></li></ol> </details> </li> <li class="toggle"> <details> <summary>Attributes</summary> <ol> <li><a href="/en-US/docs/Web/HTML/Attributes/accept">accept</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/autocomplete">autocomplete</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/capture">capture</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/crossorigin">crossorigin</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/dirname">dirname</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/disabled">disabled</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/elementtiming">elementtiming</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/for">for</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/max">max</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/maxlength">maxlength</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/min">min</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/minlength">minlength</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/multiple">multiple</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/pattern">pattern</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/placeholder">placeholder</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/readonly">readonly</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/rel">rel</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/required">required</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/size">size</a></li> <li><a href="/en-US/docs/Web/HTML/Attributes/step">step</a></li> </ol> </details> </li> <li class="toggle"> <details> <summary><code><input></code> types</summary> <ol><li><a href="/en-US/docs/Web/HTML/Element/input/button"><code><input type="button"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/checkbox"><code><input type="checkbox"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/color"><code><input type="color"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/date"><code><input type="date"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/datetime-local"><code><input type="datetime-local"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/email"><code><input type="email"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/file"><code><input type="file"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/hidden"><code><input type="hidden"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/image"><code><input type="image"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/month"><code><input type="month"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/number"><code><input type="number"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/password"><code><input type="password"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/radio"><code><input type="radio"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/range"><code><input type="range"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/reset"><code><input type="reset"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/search"><code><input type="search"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/submit"><code><input type="submit"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/tel"><code><input type="tel"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/text"><code><input type="text"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/time"><code><input type="time"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/url"><code><input type="url"></code></a></li><li><a href="/en-US/docs/Web/HTML/Element/input/week"><code><input type="week"></code></a></li></ol> </details> </li> <li class="section no-link">Guides</li> <li> <ol> <li><a href="/en-US/docs/Web/HTML/Content_categories">Content categories</a></li> <li><a href="/en-US/docs/Glossary/Block-level_content">Block-level elements</a></li> <li><a href="/en-US/docs/Glossary/Inline-level_content">Inline elements</a></li> <li><a href="/en-US/docs/Web/HTML/Quirks_Mode_and_Standards_Mode">Quirks Mode and Standards Mode</a></li> <li><a href="/en-US/docs/Web/HTML/Date_and_time_formats">Date and time formats used in HTML</a></li> <li><em><a href="/en-US/docs/Web/HTML/Constraint_validation" aria-current="page">Constraint validation</a></em></li> <li><a href="/en-US/docs/Web/HTML/Microdata">Microdata</a></li> <li><a href="/en-US/docs/Web/HTML/microformats">Microformats</a></li> <li><a href="/en-US/docs/Web/HTML/Viewport_meta_tag">Viewport meta tag</a></li> <li><a href="/en-US/docs/Web/HTML/CORS_enabled_image">Allowing cross-origin use of images and canvas</a></li> </ol> </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="#intrinsic_and_basic_constraints">Intrinsic and basic constraints</a></li><li class="document-toc-item "><a class="document-toc-link" href="#constraint_validation_process">Constraint validation process</a></li><li class="document-toc-item "><a class="document-toc-link" href="#complex_constraints_using_the_constraint_validation_api">Complex constraints using the Constraint Validation API</a></li><li class="document-toc-item "><a class="document-toc-link" href="#visual_styling_of_constraint_validation">Visual styling of constraint validation</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>Constraint validation</h1></header><div class="section-content"><p>The creation of web forms has always been a complex task. While marking up the form itself is easy, checking whether each field has a valid and coherent value is more difficult, and informing the user about the problem may become a headache. <a href="/en-US/docs/Glossary/HTML5">HTML5</a> introduced new mechanisms for forms: it added new semantic types for the <a href="/en-US/docs/Web/HTML/Element/input"><code><input></code></a> element and <em>constraint validation</em> to ease the work of checking the form content on the client side. Basic, usual constraints can be checked, without the need for JavaScript, by setting new attributes; more complex constraints can be tested using the Constraint Validation API.</p> <p>For a basic introduction to these concepts, with examples, see the <a href="/en-US/docs/Learn/Forms/Form_validation">Form validation tutorial</a>.</p> <div class="notecard note"> <p><strong>Note:</strong> HTML Constraint validation doesn't remove the need for validation on the <em>server side</em>. Even though far fewer invalid form requests are to be expected, invalid ones can still be sent in many ways:</p> <ul> <li>By modifying HTML via the browser's developer tools.</li> <li>By hand-crafting an HTTP request without using the form.</li> <li>By programmatically writing content into the form (certain constraint validations are <em>only run</em> for user input, and not if you set the value of a form field using JavaScript).</li> </ul> <p>Therefore, you should always validate form data on the server side, consistent with what is done on the client side.</p> </div></div><section aria-labelledby="intrinsic_and_basic_constraints"><h2 id="intrinsic_and_basic_constraints"><a href="#intrinsic_and_basic_constraints">Intrinsic and basic constraints</a></h2><div class="section-content"><p>In HTML, basic constraints are declared in two ways:</p> <ul> <li>By choosing the most semantically appropriate value for the <a href="/en-US/docs/Web/HTML/Element/input#type"><code>type</code></a> attribute of the <a href="/en-US/docs/Web/HTML/Element/input"><code><input></code></a> element, e.g., choosing the <code>email</code> type automatically creates a constraint that checks whether the value is a valid email address.</li> <li>By setting values on validation-related attributes, allowing basic constraints to be described without the need for JavaScript.</li> </ul></div></section><section aria-labelledby="semantic_input_types"><h3 id="semantic_input_types"><a href="#semantic_input_types">Semantic input types</a></h3><div class="section-content"><p>The intrinsic constraints for the <a href="/en-US/docs/Web/HTML/Element/input#type"><code>type</code></a> attribute are:</p> <figure class="table-container"><table> <thead> <tr> <th>Input type</th> <th>Constraint description</th> <th>Associated violation</th> </tr> </thead> <tbody> <tr> <td><a href="/en-US/docs/Web/HTML/Element/input/url"><code><input type="URL"></code></a></td> <td>The value must be an absolute <a href="/en-US/docs/Learn/Common_questions/Web_mechanics/What_is_a_URL">URL</a>, as defined in the <a href="https://url.spec.whatwg.org/" class="external" target="_blank">URL Living Standard</a>.</td> <td><strong><a href="/en-US/docs/Web/API/ValidityState/typeMismatch">TypeMismatch</a></strong> constraint violation</td> </tr> <tr> <td><a href="/en-US/docs/Web/HTML/Element/input/email"><code><input type="email"></code></a></td> <td>The value must be a syntactically valid email address, which generally has the format <code>username@hostname.tld</code> but can also be local such as <code>username@hostname</code>.</td> <td><strong><a href="/en-US/docs/Web/API/ValidityState/typeMismatch">TypeMismatch</a></strong> constraint violation</td> </tr> </tbody> </table></figure> <p>For both of these input types, if the <a href="/en-US/docs/Web/HTML/Element/input#multiple"><code>multiple</code></a> attribute is set, several values can be set, as a comma-separated list. If any of these do not satisfy the condition described here, the <strong>Type mismatch</strong> constraint violation is triggered.</p> <p>Note that most input types don't have intrinsic constraints, as some are barred from constraint validation or have a sanitization algorithm transforming incorrect values to a correct default.</p></div></section><section aria-labelledby="validation-related_attributes"><h3 id="validation-related_attributes"><a href="#validation-related_attributes">Validation-related attributes</a></h3><div class="section-content"><p>In addition to the <code>type</code> attribute described above, the following attributes are used to describe basic constraints:</p> <figure class="table-container"><table class="standard-table"> <thead> <tr> <th scope="col">Attribute</th> <th scope="col">Input types supporting the attribute</th> <th scope="col">Possible values</th> <th scope="col">Constraint description</th> <th scope="col">Associated violation</th> </tr> </thead> <tbody> <tr> <td><code><a href="/en-US/docs/Web/HTML/Attributes/pattern">pattern</a></code></td> <td> <code>text</code>, <code>search</code>, <code>url</code>, <code>tel</code>, <code>email</code>, <code>password</code> </td> <td> A <a href="/en-US/docs/Web/JavaScript/Guide/Regular_expressions">JavaScript regular expression</a> (compiled with the <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/global"><code>global</code></a>, <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/ignoreCase"><code>ignoreCase</code></a>, and <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/multiline"><code>multiline</code></a> flags <em>disabled</em>) </td> <td>The value must match the pattern.</td> <td> <a href="/en-US/docs/Web/API/ValidityState/patternMismatch"><strong><code>patternMismatch</code></strong></a> constraint violation </td> </tr> <tr> <td rowspan="3"><code><a href="/en-US/docs/Web/HTML/Attributes/min">min</a></code></td> <td><code>range</code>, <code>number</code></td> <td>A valid number</td> <td rowspan="3">The value must be greater than or equal to the value.</td> <td rowspan="3"> <strong><code><a href="/en-US/docs/Web/API/ValidityState/rangeUnderflow">rangeUnderflow</a></code></strong> constraint violation </td> </tr> <tr> <td><code>date</code>, <code>month</code>, <code>week</code></td> <td>A valid date</td> </tr> <tr> <td><code>datetime-local</code>, <code>time</code></td> <td>A valid date and time</td> </tr> <tr> <td rowspan="3"><code><a href="/en-US/docs/Web/HTML/Attributes/max">max</a></code></td> <td><code>range</code>, <code>number</code></td> <td>A valid number</td> <td rowspan="3">The value must be less than or equal to the value</td> <td rowspan="3"> <strong><code><a href="/en-US/docs/Web/API/ValidityState/rangeOverflow">rangeOverflow</a></code></strong> constraint violation </td> </tr> <tr> <td><code>date</code>, <code>month</code>, <code>week</code></td> <td>A valid date</td> </tr> <tr> <td><code>datetime-local</code>, <code>time</code></td> <td>A valid date and time</td> </tr> <tr> <td><code><a href="/en-US/docs/Web/HTML/Attributes/required">required</a></code></td> <td> <code>text</code>, <code>search</code>, <code>url</code>, <code>tel</code>, <code>email</code>, <code>password</code>, <code>date</code>, <code>datetime-local</code>, <code>month</code>, <code>week</code>, <code>time</code>, <code>number</code>, <code>checkbox</code>, <code>radio</code>, <code>file</code>; also on the <a href="/en-US/docs/Web/HTML/Element/select"><code><select></code></a> and <a href="/en-US/docs/Web/HTML/Element/textarea"><code><textarea></code></a> elements </td> <td> <em>none</em> as it is a Boolean attribute: its presence means <em>true</em>, its absence means <em>false</em> </td> <td>There must be a value (if set).</td> <td> <strong><code><a href="/en-US/docs/Web/API/ValidityState/valueMissing">valueMissing</a></code></strong> constraint violation </td> </tr> <tr> <td rowspan="5"><code><a href="/en-US/docs/Web/HTML/Attributes/step">step</a></code></td> <td><code>date</code></td> <td>An integer number of days</td> <td rowspan="5"> Unless the step is set to the <code>any</code> literal, the value must be <strong>min</strong> + an integral multiple of the step. </td> <td rowspan="5"> <strong><code><a href="/en-US/docs/Web/API/ValidityState/stepMismatch">stepMismatch</a></code></strong> constraint violation </td> </tr> <tr> <td><code>month</code></td> <td>An integer number of months</td> </tr> <tr> <td><code>week</code></td> <td>An integer number of weeks</td> </tr> <tr> <td><code>datetime-local</code>, <code>time</code></td> <td>An integer number of seconds</td> </tr> <tr> <td><code>range</code>, <code>number</code></td> <td>An integer</td> </tr> <tr> <td><code><a href="/en-US/docs/Web/HTML/Attributes/minlength">minlength</a></code></td> <td> <code>text</code>, <code>search</code>, <code>url</code>, <code>tel</code>, <code>email</code>, <code>password</code>; also on the <a href="/en-US/docs/Web/HTML/Element/textarea"><code><textarea></code></a> element </td> <td>An integer length</td> <td> The number of characters (code points) must not be less than the value of the attribute, if non-empty. All newlines are normalized to a single character (as opposed to CRLF pairs) for <a href="/en-US/docs/Web/HTML/Element/textarea"><code><textarea></code></a>. </td> <td> <strong><code><a href="/en-US/docs/Web/API/ValidityState/tooShort">tooShort</a></code></strong> constraint violation </td> </tr> <tr> <td><code><a href="/en-US/docs/Web/HTML/Attributes/maxlength">maxlength</a></code></td> <td> <code>text</code>, <code>search</code>, <code>url</code>, <code>tel</code>, <code>email</code>, <code>password</code>; also on the <a href="/en-US/docs/Web/HTML/Element/textarea"><code><textarea></code></a> element </td> <td>An integer length</td> <td> The number of characters (code points) must not exceed the value of the attribute. </td> <td> <strong><code><a href="/en-US/docs/Web/API/ValidityState/tooLong">tooLong</a></code></strong> constraint violation </td> </tr> </tbody> </table></figure></div></section><section aria-labelledby="constraint_validation_process"><h2 id="constraint_validation_process"><a href="#constraint_validation_process">Constraint validation process</a></h2><div class="section-content"><p>Constraint validation is done through the Constraint Validation API either on a single form element or at the form level, on the <a href="/en-US/docs/Web/HTML/Element/form"><code><form></code></a> element itself. The constraint validation is done in the following ways:</p> <ul> <li>By a call to the <code>checkValidity()</code> or <code>reportValidity()</code> method of a form-associated DOM interface, (<a href="/en-US/docs/Web/API/HTMLInputElement"><code>HTMLInputElement</code></a>, <a href="/en-US/docs/Web/API/HTMLSelectElement"><code>HTMLSelectElement</code></a>, <a href="/en-US/docs/Web/API/HTMLButtonElement"><code>HTMLButtonElement</code></a>, <a href="/en-US/docs/Web/API/HTMLOutputElement"><code>HTMLOutputElement</code></a> or <a href="/en-US/docs/Web/API/HTMLTextAreaElement"><code>HTMLTextAreaElement</code></a>), which evaluates the constraints only on this element, allowing a script to get this information. The <code>checkValidity()</code> method returns a Boolean indicating whether the element's value passes its constraints. (This is typically done by the user-agent when determining which of the CSS pseudo-classes, <a href="/en-US/docs/Web/CSS/:valid"><code>:valid</code></a> or <a href="/en-US/docs/Web/CSS/:invalid"><code>:invalid</code></a>, applies.) In contrast, the <code>reportValidity()</code> method reports any constraint failures to the user.</li> <li>By a call to the <code>checkValidity()</code> or <code>reportValidity()</code> method on the <a href="/en-US/docs/Web/API/HTMLFormElement"><code>HTMLFormElement</code></a> interface.</li> <li>By submitting the form itself.</li> </ul> <p>Calling <code>checkValidity()</code> is called <em>statically</em> validating the constraints, while calling <code>reportValidity()</code> or submitting the form is called <em>interactively</em> validating the constraints.</p> <div class="notecard note"> <p><strong>Note:</strong></p> <ul> <li>If the <a href="/en-US/docs/Web/HTML/Element/form#novalidate"><code>novalidate</code></a> attribute is set on the <a href="/en-US/docs/Web/HTML/Element/form"><code><form></code></a> element, interactive validation of the constraints doesn't happen.</li> <li>Calling the <code>submit()</code> method on the <a href="/en-US/docs/Web/API/HTMLFormElement"><code>HTMLFormElement</code></a> interface doesn't trigger a constraint validation. In other words, this method sends the form data to the server even if it doesn't satisfy the constraints. Call the <code>click()</code> method on a submit button instead.</li> <li>The <code>minlength</code> and <code>maxlength</code> constraints are only checked on user-provided input. They are not checked if a value is set programmatically, even when explicitly calling <code>checkValidity()</code> or <code>reportValidity()</code>.</li> </ul> </div></div></section><section aria-labelledby="complex_constraints_using_the_constraint_validation_api"><h2 id="complex_constraints_using_the_constraint_validation_api"><a href="#complex_constraints_using_the_constraint_validation_api">Complex constraints using the Constraint Validation API</a></h2><div class="section-content"><p>Using JavaScript and the Constraint API, it is possible to implement more complex constraints, for example, constraints combining several fields, or constraints involving complex calculations.</p> <p>Basically, the idea is to trigger JavaScript on some form field event (like <strong>onchange</strong>) to calculate whether the constraint is violated, and then to use the method <code>field.setCustomValidity()</code> to set the result of the validation: an empty string means the constraint is satisfied, and any other string means there is an error and this string is the error message to display to the user.</p></div></section><section aria-labelledby="constraint_combining_several_fields_postal_code_validation"><h3 id="constraint_combining_several_fields_postal_code_validation"><a href="#constraint_combining_several_fields_postal_code_validation">Constraint combining several fields: Postal code validation</a></h3><div class="section-content"><p>The postal code format varies from one country to another. Not only do most countries allow an optional prefix with the country code (like <code>D-</code> in Germany, <code>F-</code> in France or Switzerland), but some countries have postal codes with only a fixed number of digits; others, like the UK, have more complex structures, allowing letters at some specific positions.</p> <div class="notecard note"> <p><strong>Note:</strong> This is not a comprehensive postal code validation library, but rather a demonstration of the key concepts.</p> </div> <p>As an example, we will add a script checking the constraint validation for a form:</p> <div class="code-example"><div class="example-header"><span class="language-name">html</span></div><pre class="brush: html notranslate"><code><form> <label for="postal-code">Postal Code: </label> <input type="text" id="postal-code" /> <label for="country">Country: </label> <select id="country"> <option value="ch">Switzerland</option> <option value="fr">France</option> <option value="de">Germany</option> <option value="nl">The Netherlands</option> </select> <input type="submit" value="Validate" /> </form> </code></pre></div> <p>This displays the following form:</p><div class="code-example"><div class="example-header"></div><iframe class="sample-code-frame" title="Constraint combining several fields Postal code validation sample" id="frame_constraint_combining_several_fields_postal_code_validation" src="about:blank" data-live-path="/en-US/docs/Web/HTML/Constraint_validation" data-live-id="constraint_combining_several_fields_postal_code_validation" sandbox="allow-same-origin allow-scripts" loading="lazy"></iframe></div> <p>First, we write a function checking the constraint itself:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>function checkPostalCode() { // For each country, defines the pattern that the postal code has to follow const constraints = { ch: [ "^(CH-)?\\d{4}$", "Swiss postal codes must have exactly 4 digits: e.g. CH-1950 or 1950", ], fr: [ "^(F-)?\\d{5}$", "French postal codes must have exactly 5 digits: e.g. F-75012 or 75012", ], de: [ "^(D-)?\\d{5}$", "German postal codes must have exactly 5 digits: e.g. D-12345 or 12345", ], nl: [ "^(NL-)?\\d{4}\\s*([A-RT-Z][A-Z]|S[BCE-RT-Z])$", "Dutch postal codes must have exactly 4 digits, followed by 2 letters except SA, SD and SS", ], }; // Read the country id const country = document.getElementById("country").value; // Get the NPA field const postalCodeField = document.getElementById("postal-code"); // Build the constraint checker const constraint = new RegExp(constraints[country][0], ""); console.log(constraint); // Check it! if (constraint.test(postalCodeField.value)) { // The postal code follows the constraint, we use the ConstraintAPI to tell it postalCodeField.setCustomValidity(""); } else { // The postal code doesn't follow the constraint, we use the ConstraintAPI to // give a message about the format required for this country postalCodeField.setCustomValidity(constraints[country][1]); } } </code></pre></div> <p>Then we link it to the <strong>onchange</strong> event for the <a href="/en-US/docs/Web/HTML/Element/select"><code><select></code></a> and the <strong>oninput</strong> event for the <a href="/en-US/docs/Web/HTML/Element/input"><code><input></code></a>:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>window.onload = () => { document.getElementById("country").onchange = checkPostalCode; document.getElementById("postal-code").oninput = checkPostalCode; }; </code></pre></div></div></section><section aria-labelledby="limiting_the_size_of_a_file_before_its_upload"><h3 id="limiting_the_size_of_a_file_before_its_upload"><a href="#limiting_the_size_of_a_file_before_its_upload">Limiting the size of a file before its upload</a></h3><div class="section-content"><p>Another common constraint is to limit the size of a file to be uploaded. Checking this on the client side before the file is transmitted to the server requires combining the Constraint Validation API, and especially the <code>field.setCustomValidity()</code> method, with another JavaScript API, here the File API.</p> <p>Here is the HTML part:</p> <div class="code-example"><div class="example-header"><span class="language-name">html</span></div><pre class="brush: html notranslate"><code><label for="fs">Select a file smaller than 75 kB: </label> <input type="file" id="fs" /> </code></pre></div> <p>This displays:</p><div class="code-example"><div class="example-header"></div><iframe class="sample-code-frame" title="Limiting the size of a file before its upload sample" id="frame_limiting_the_size_of_a_file_before_its_upload" src="about:blank" data-live-path="/en-US/docs/Web/HTML/Constraint_validation" data-live-id="limiting_the_size_of_a_file_before_its_upload" sandbox="allow-same-origin allow-scripts" loading="lazy"></iframe></div> <p>The JavaScript reads the file selected, uses the <code>File.size()</code> method to get its size, compares it to the (hard coded) limit, and calls the Constraint API to inform the browser if there is a violation:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>function checkFileSize() { const fs = document.getElementById("fs"); const files = fs.files; // If there is (at least) one file selected if (files.length > 0) { if (files[0].size > 75 * 1000) { // Check the constraint fs.setCustomValidity("The selected file must not be larger than 75 kB"); fs.reportValidity(); return; } } // No custom constraint violation fs.setCustomValidity(""); } </code></pre></div> <p>Finally, we hook the method with the correct event:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>window.onload = () => { document.getElementById("fs").onchange = checkFileSize; }; </code></pre></div></div></section><section aria-labelledby="visual_styling_of_constraint_validation"><h2 id="visual_styling_of_constraint_validation"><a href="#visual_styling_of_constraint_validation">Visual styling of constraint validation</a></h2><div class="section-content"><p>Apart from setting constraints, web developers want to control what messages are displayed to the users and how they are styled.</p></div></section><section aria-labelledby="controlling_the_look_of_elements"><h3 id="controlling_the_look_of_elements"><a href="#controlling_the_look_of_elements">Controlling the look of elements</a></h3><div class="section-content"><p>The look of elements can be controlled via CSS pseudo-classes.</p> <h4 id="required_and_optional_css_pseudo-classes">:required and :optional CSS pseudo-classes</h4> <p>The <a href="/en-US/docs/Web/CSS/:required"><code>:required</code></a> and <a href="/en-US/docs/Web/CSS/:optional"><code>:optional</code></a> <a href="/en-US/docs/Web/CSS/Pseudo-classes">pseudo-classes</a> allow writing selectors that match form elements that have the <a href="/en-US/docs/Web/HTML/Element/input#required"><code>required</code></a> attribute, or that don't have it.</p> <h4 id="placeholder-shown_css_pseudo-class">:placeholder-shown CSS pseudo-class</h4> <p>See <a href="/en-US/docs/Web/CSS/:placeholder-shown"><code>:placeholder-shown</code></a>.</p> <h4 id="valid_invalid_css_pseudo-classes">:valid :invalid CSS pseudo-classes</h4> <p>The <a href="/en-US/docs/Web/CSS/:valid"><code>:valid</code></a> and <a href="/en-US/docs/Web/CSS/:invalid"><code>:invalid</code></a> <a href="/en-US/docs/Web/CSS/Pseudo-classes">pseudo-classes</a> are used to represent <input> elements whose content validates and fails to validate respectively according to the input's type setting. These classes allow the user to style valid or invalid form elements to make it easier to identify elements that are either formatted correctly or incorrectly.</p></div></section><section aria-labelledby="controlling_the_text_of_constraint_violation"><h3 id="controlling_the_text_of_constraint_violation"><a href="#controlling_the_text_of_constraint_violation">Controlling the text of constraint violation</a></h3><div class="section-content"><p>The following items can help with controlling the text of a constraint violation:</p> <ul> <li> <p>The <code>setCustomValidity(message)</code> method on the following elements:</p> <ul> <li><a href="/en-US/docs/Web/HTML/Element/fieldset"><code><fieldset></code></a>. Note: Setting a custom validity message on fieldset elements will not prevent form submission in most browsers.</li> <li><a href="/en-US/docs/Web/HTML/Element/input"><code><input></code></a></li> <li><a href="/en-US/docs/Web/HTML/Element/output"><code><output></code></a></li> <li><a href="/en-US/docs/Web/HTML/Element/select"><code><select></code></a></li> <li>Submit buttons (created with either a <a href="/en-US/docs/Web/HTML/Element/button"><code><button></code></a> element with the <code>submit</code> type, or an <code>input</code> element with the <a href="/en-US/docs/Web/HTML/Element/input/submit">submit</a> type. Other types of buttons do not participate in constraint validation.</li> <li><a href="/en-US/docs/Web/HTML/Element/textarea"><code><textarea></code></a></li> </ul> </li> <li> <p>The <a href="/en-US/docs/Web/API/ValidityState"><code>ValidityState</code></a> interface describes the object returned by the <code>validity</code> property of the element types listed above. It represents various ways that an entered value can be invalid. 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2.512 0 4.812 1.522 4.812 4.84v6.402h1.863v3.044h-.005Zm9.036.307c-4.314 0-7.296-2.635-7.296-7.106 0-4.096 2.484-7.481 7.514-7.481s7.481 3.38 7.481 7.29c0 4.472-3.228 7.297-7.699 7.297Zm22.578-.307H51.942l-.403-2.11 7.7-8.846h-4.376l-.621 2.17-2.888-.313.498-4.907h12.294l.313 2.11-7.767 8.852h4.533l.654-2.172 3.167.308-.872 4.908Zm7.99 0h-4.191v-5.03h4.19v5.03Zm0-8.976h-4.191v-5.03h4.19v5.03Zm2.618 8.976 6.054-21.358h3.945l-6.054 21.358h-3.945Zm8.136 0 6.048-21.358h3.945l-6.054 21.358h-3.939Zm21.486.307c-1.863 0-2.887-1.085-3.072-2.792-.805 1.427-2.232 2.792-4.498 2.792-2.02 0-4.314-1.085-4.314-4.006 0-3.447 3.323-4.253 6.518-4.253.778 0 1.584.034 2.3.124v-.465c0-1.427-.034-3.133-2.3-3.133-.84 0-1.488.061-2.143.402l-.453 1.578-3.195-.34.549-3.224c2.45-.996 3.692-1.27 5.992-1.27 3.01 0 5.556 1.55 5.556 4.75v6.083c0 .805.314 1.085.963 1.085.184 0 .375-.034.587-.095l.034 2.11a5.432 5.432 0 0 1-2.524.654Z" fill="currentColor"></path></svg></a><ul class="footer-moz-list"><li 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href="https://foundation.mozilla.org/">Mozilla Foundation</a>.<br/>Portions of this content are ©1998–2024 by individual mozilla.org contributors. 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/HTML/Constraint_validation","doc":{"isMarkdown":true,"isTranslated":false,"isActive":true,"flaws":{},"title":"Constraint validation","mdn_url":"/en-US/docs/Web/HTML/Constraint_validation","locale":"en-US","native":"English (US)","sidebarHTML":"\n <ol>\n <li class=\"section\"><a href=\"/en-US/docs/Web/HTML\">HTML</a></li>\n <li class=\"section\"><a href=\"/en-US/docs/Learn/HTML\">Tutorials</a></li>\n <li><a href=\"/en-US/docs/Learn/Getting_started_with_the_web/HTML_basics\">HTML basics</a></li>\n <li class=\"toggle\">\n <details>\n <summary>Introduction to HTML</summary>\n <ol>\n <li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML\">Introduction to HTML overview</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Getting_started\">Getting started with HTML</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/The_head_metadata_in_HTML\">What's in the head? Metadata in HTML</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/HTML_text_fundamentals\">HTML text fundamentals</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Creating_hyperlinks\">Creating hyperlinks</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Advanced_text_formatting\">Advanced text formatting</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Document_and_website_structure\">Document and website structure</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Debugging_HTML\">Debugging HTML</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Marking_up_a_letter\">Assessment: Marking up a letter</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Structuring_a_page_of_content\">Assessment: Structuring a page of content</a></li>\n </ol>\n </details>\n </li>\n <li class=\"toggle\">\n <details>\n <summary>Multimedia and embedding</summary>\n <ol>\n <li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding\">Multimedia and embedding overview</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Images_in_HTML\">Images in HTML</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Video_and_audio_content\">Video and audio content</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Other_embedding_technologies\">From object to iframe — other embedding technologies</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Adding_vector_graphics_to_the_Web\">Adding vector graphics to the web</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Responsive_images\">Responsive images</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Mozilla_splash_page\">Assessment: Mozilla splash page</a></li>\n </ol>\n </details>\n </li>\n <li class=\"toggle\">\n <details>\n <summary>HTML tables</summary>\n <ol>\n <li><a href=\"/en-US/docs/Learn/HTML/Tables\">HTML tables overview</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Tables/Basics\">HTML table basics</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Tables/Advanced\">HTML table advanced features and accessibility</a></li>\n <li><a href=\"/en-US/docs/Learn/HTML/Tables/Structuring_planet_data\">Assessment: Structuring planet data</a></li>\n </ol>\n </details>\n </li>\n <li class=\"section\"><a href=\"/en-US/docs/Web/HTML/Reference\">References</a></li>\n <li class=\"toggle\">\n <details>\n <summary>HTML elements</summary>\n <ol><li><a href=\"/en-US/docs/Web/HTML/Element/a\"><code><a></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/abbr\"><code><abbr></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/acronym\"><code><acronym></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/HTML/Element/address\"><code><address></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/area\"><code><area></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/article\"><code><article></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/aside\"><code><aside></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/audio\"><code><audio></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/b\"><code><b></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/base\"><code><base></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/bdi\"><code><bdi></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/bdo\"><code><bdo></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/big\"><code><big></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/HTML/Element/blockquote\"><code><blockquote></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/body\"><code><body></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/br\"><code><br></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/button\"><code><button></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/canvas\"><code><canvas></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/caption\"><code><caption></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/center\"><code><center></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/HTML/Element/cite\"><code><cite></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/code\"><code><code></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/col\"><code><col></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/colgroup\"><code><colgroup></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/data\"><code><data></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/datalist\"><code><datalist></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/dd\"><code><dd></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/del\"><code><del></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/details\"><code><details></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/dfn\"><code><dfn></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/dialog\"><code><dialog></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/dir\"><code><dir></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/HTML/Element/div\"><code><div></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/dl\"><code><dl></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/dt\"><code><dt></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/em\"><code><em></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/embed\"><code><embed></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/fencedframe\"><code><fencedframe></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/HTML/Element/fieldset\"><code><fieldset></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/figcaption\"><code><figcaption></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/figure\"><code><figure></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/font\"><code><font></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/HTML/Element/footer\"><code><footer></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/form\"><code><form></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/frame\"><code><frame></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/HTML/Element/frameset\"><code><frameset></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/HTML/Element/Heading_Elements\"><code><h1></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/head\"><code><head></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/header\"><code><header></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/hgroup\"><code><hgroup></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/hr\"><code><hr></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/html\"><code><html></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/i\"><code><i></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/iframe\"><code><iframe></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/img\"><code><img></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input\"><code><input></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/ins\"><code><ins></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/kbd\"><code><kbd></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/label\"><code><label></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/legend\"><code><legend></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/li\"><code><li></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/link\"><code><link></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/main\"><code><main></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/map\"><code><map></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/mark\"><code><mark></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/marquee\"><code><marquee></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/HTML/Element/menu\"><code><menu></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/meta\"><code><meta></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/meter\"><code><meter></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/nav\"><code><nav></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/nobr\"><code><nobr></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/HTML/Element/noembed\"><code><noembed></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/HTML/Element/noframes\"><code><noframes></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/HTML/Element/noscript\"><code><noscript></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/object\"><code><object></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/ol\"><code><ol></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/optgroup\"><code><optgroup></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/option\"><code><option></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/output\"><code><output></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/p\"><code><p></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/param\"><code><param></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/HTML/Element/picture\"><code><picture></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/plaintext\"><code><plaintext></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/HTML/Element/portal\"><code><portal></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/HTML/Element/pre\"><code><pre></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/progress\"><code><progress></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/q\"><code><q></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/rb\"><code><rb></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/HTML/Element/rp\"><code><rp></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/rt\"><code><rt></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/rtc\"><code><rtc></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/HTML/Element/ruby\"><code><ruby></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/s\"><code><s></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/samp\"><code><samp></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/script\"><code><script></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/search\"><code><search></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/section\"><code><section></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/select\"><code><select></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/slot\"><code><slot></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/small\"><code><small></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/source\"><code><source></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/span\"><code><span></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/strike\"><code><strike></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/HTML/Element/strong\"><code><strong></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/style\"><code><style></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/sub\"><code><sub></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/summary\"><code><summary></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/sup\"><code><sup></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/table\"><code><table></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/tbody\"><code><tbody></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/td\"><code><td></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/template\"><code><template></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/textarea\"><code><textarea></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/tfoot\"><code><tfoot></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/th\"><code><th></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/thead\"><code><thead></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/time\"><code><time></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/title\"><code><title></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/tr\"><code><tr></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/track\"><code><track></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/tt\"><code><tt></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/HTML/Element/u\"><code><u></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/ul\"><code><ul></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/var\"><code><var></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/video\"><code><video></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/wbr\"><code><wbr></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/xmp\"><code><xmp></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></ol>\n </details>\n </li>\n <li class=\"toggle\">\n <details>\n <summary>Global attributes</summary>\n <ol><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/accesskey\"><code>accesskey</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/anchor\"><code>anchor</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/HTML/Global_attributes/autocapitalize\"><code>autocapitalize</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/autocorrect\"><code>autocorrect</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/HTML/Global_attributes/autofocus\"><code>autofocus</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/class\"><code>class</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/contenteditable\"><code>contenteditable</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/data-*\"><code>data-*</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/dir\"><code>dir</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/draggable\"><code>draggable</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/enterkeyhint\"><code>enterkeyhint</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/exportparts\"><code>exportparts</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/hidden\"><code>hidden</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/id\"><code>id</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/inert\"><code>inert</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/inputmode\"><code>inputmode</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/is\"><code>is</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/itemid\"><code>itemid</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/itemprop\"><code>itemprop</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/itemref\"><code>itemref</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/itemscope\"><code>itemscope</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/itemtype\"><code>itemtype</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/lang\"><code>lang</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/nonce\"><code>nonce</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/part\"><code>part</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/popover\"><code>popover</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/slot\"><code>slot</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/spellcheck\"><code>spellcheck</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/style\"><code>style</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/tabindex\"><code>tabindex</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/title\"><code>title</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/translate\"><code>translate</code></a></li><li><a href=\"/en-US/docs/Web/HTML/Global_attributes/virtualkeyboardpolicy\"><code>virtualkeyboardpolicy</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/HTML/Global_attributes/writingsuggestions\"><code>writingsuggestions</code></a></li></ol>\n </details>\n </li>\n <li class=\"toggle\">\n <details>\n <summary>Attributes</summary>\n <ol>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/accept\">accept</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/autocomplete\">autocomplete</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/capture\">capture</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/crossorigin\">crossorigin</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/dirname\">dirname</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/disabled\">disabled</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/elementtiming\">elementtiming</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/for\">for</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/max\">max</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/maxlength\">maxlength</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/min\">min</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/minlength\">minlength</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/multiple\">multiple</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/pattern\">pattern</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/placeholder\">placeholder</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/readonly\">readonly</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/rel\">rel</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/required\">required</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/size\">size</a></li>\n \n <li><a href=\"/en-US/docs/Web/HTML/Attributes/step\">step</a></li>\n \n </ol>\n </details>\n </li>\n <li class=\"toggle\">\n <details>\n <summary><code><input></code> types</summary>\n <ol><li><a href=\"/en-US/docs/Web/HTML/Element/input/button\"><code><input type=\"button\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/checkbox\"><code><input type=\"checkbox\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/color\"><code><input type=\"color\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/date\"><code><input type=\"date\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/datetime-local\"><code><input type=\"datetime-local\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/email\"><code><input type=\"email\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/file\"><code><input type=\"file\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/hidden\"><code><input type=\"hidden\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/image\"><code><input type=\"image\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/month\"><code><input type=\"month\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/number\"><code><input type=\"number\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/password\"><code><input type=\"password\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/radio\"><code><input type=\"radio\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/range\"><code><input type=\"range\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/reset\"><code><input type=\"reset\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/search\"><code><input type=\"search\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/submit\"><code><input type=\"submit\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/tel\"><code><input type=\"tel\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/text\"><code><input type=\"text\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/time\"><code><input type=\"time\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/url\"><code><input type=\"url\"></code></a></li><li><a href=\"/en-US/docs/Web/HTML/Element/input/week\"><code><input type=\"week\"></code></a></li></ol>\n </details>\n </li>\n <li class=\"section no-link\">Guides</li>\n <li>\n <ol>\n <li><a href=\"/en-US/docs/Web/HTML/Content_categories\">Content categories</a></li>\n <li><a href=\"/en-US/docs/Glossary/Block-level_content\">Block-level elements</a></li>\n <li><a href=\"/en-US/docs/Glossary/Inline-level_content\">Inline elements</a></li>\n <li><a href=\"/en-US/docs/Web/HTML/Quirks_Mode_and_Standards_Mode\">Quirks Mode and Standards Mode</a></li>\n <li><a href=\"/en-US/docs/Web/HTML/Date_and_time_formats\">Date and time formats used in HTML</a></li>\n <li><em><a href=\"/en-US/docs/Web/HTML/Constraint_validation\" aria-current=\"page\">Constraint validation</a></em></li>\n <li><a href=\"/en-US/docs/Web/HTML/Microdata\">Microdata</a></li>\n <li><a href=\"/en-US/docs/Web/HTML/microformats\">Microformats</a></li>\n <li><a href=\"/en-US/docs/Web/HTML/Viewport_meta_tag\">Viewport meta tag</a></li>\n <li><a href=\"/en-US/docs/Web/HTML/CORS_enabled_image\">Allowing cross-origin use of images and canvas</a></li>\n </ol>\n </li>\n </ol>\n","sidebarMacro":"HTMLSidebar","body":[{"type":"prose","value":{"id":null,"title":null,"isH3":false,"content":"<p>The creation of web forms has always been a complex task. While marking up the form itself is easy, checking whether each field has a valid and coherent value is more difficult, and informing the user about the problem may become a headache. <a href=\"/en-US/docs/Glossary/HTML5\">HTML5</a> introduced new mechanisms for forms: it added new semantic types for the <a href=\"/en-US/docs/Web/HTML/Element/input\"><code><input></code></a> element and <em>constraint validation</em> to ease the work of checking the form content on the client side. Basic, usual constraints can be checked, without the need for JavaScript, by setting new attributes; more complex constraints can be tested using the Constraint Validation API.</p>\n<p>For a basic introduction to these concepts, with examples, see the <a href=\"/en-US/docs/Learn/Forms/Form_validation\">Form validation tutorial</a>.</p>\n<div class=\"notecard note\">\n <p><strong>Note:</strong> HTML Constraint validation doesn't remove the need for validation on the <em>server side</em>. Even though far fewer invalid form requests are to be expected, invalid ones can still be sent in many ways:</p>\n <ul>\n <li>By modifying HTML via the browser's developer tools.</li>\n <li>By hand-crafting an HTTP request without using the form.</li>\n <li>By programmatically writing content into the form (certain constraint validations are <em>only run</em> for user input, and not if you set the value of a form field using JavaScript).</li>\n </ul>\n <p>Therefore, you should always validate form data on the server side, consistent with what is done on the client side.</p>\n</div>"}},{"type":"prose","value":{"id":"intrinsic_and_basic_constraints","title":"Intrinsic and basic constraints","isH3":false,"content":"<p>In HTML, basic constraints are declared in two ways:</p>\n<ul>\n <li>By choosing the most semantically appropriate value for the <a href=\"/en-US/docs/Web/HTML/Element/input#type\"><code>type</code></a> attribute of the <a href=\"/en-US/docs/Web/HTML/Element/input\"><code><input></code></a> element, e.g., choosing the <code>email</code> type automatically creates a constraint that checks whether the value is a valid email address.</li>\n <li>By setting values on validation-related attributes, allowing basic constraints to be described without the need for JavaScript.</li>\n</ul>"}},{"type":"prose","value":{"id":"semantic_input_types","title":"Semantic input types","isH3":true,"content":"<p>The intrinsic constraints for the <a href=\"/en-US/docs/Web/HTML/Element/input#type\"><code>type</code></a> attribute are:</p>\n<figure class=\"table-container\"><table>\n <thead>\n <tr>\n <th>Input type</th>\n <th>Constraint description</th>\n <th>Associated violation</th>\n </tr>\n </thead>\n <tbody>\n <tr>\n <td><a href=\"/en-US/docs/Web/HTML/Element/input/url\"><code><input type=\"URL\"></code></a></td>\n <td>The value must be an absolute <a href=\"/en-US/docs/Learn/Common_questions/Web_mechanics/What_is_a_URL\">URL</a>, as defined in the <a href=\"https://url.spec.whatwg.org/\" class=\"external\" target=\"_blank\">URL Living Standard</a>.</td>\n <td><strong><a href=\"/en-US/docs/Web/API/ValidityState/typeMismatch\">TypeMismatch</a></strong> constraint violation</td>\n </tr>\n <tr>\n <td><a href=\"/en-US/docs/Web/HTML/Element/input/email\"><code><input type=\"email\"></code></a></td>\n <td>The value must be a syntactically valid email address, which generally has the format <code>username@hostname.tld</code> but can also be local such as <code>username@hostname</code>.</td>\n <td><strong><a href=\"/en-US/docs/Web/API/ValidityState/typeMismatch\">TypeMismatch</a></strong> constraint violation</td>\n </tr>\n </tbody>\n</table></figure>\n<p>For both of these input types, if the <a href=\"/en-US/docs/Web/HTML/Element/input#multiple\"><code>multiple</code></a> attribute is set, several values can be set, as a comma-separated list. If any of these do not satisfy the condition described here, the <strong>Type mismatch</strong> constraint violation is triggered.</p>\n<p>Note that most input types don't have intrinsic constraints, as some are barred from constraint validation or have a sanitization algorithm transforming incorrect values to a correct default.</p>"}},{"type":"prose","value":{"id":"validation-related_attributes","title":"Validation-related attributes","isH3":true,"content":"<p>In addition to the <code>type</code> attribute described above, the following attributes are used to describe basic constraints:</p>\n<figure class=\"table-container\"><table class=\"standard-table\">\n <thead>\n <tr>\n <th scope=\"col\">Attribute</th>\n <th scope=\"col\">Input types supporting the attribute</th>\n <th scope=\"col\">Possible values</th>\n <th scope=\"col\">Constraint description</th>\n <th scope=\"col\">Associated violation</th>\n </tr>\n </thead>\n <tbody>\n <tr>\n <td><code><a href=\"/en-US/docs/Web/HTML/Attributes/pattern\">pattern</a></code></td>\n <td>\n <code>text</code>, <code>search</code>, <code>url</code>,\n <code>tel</code>, <code>email</code>, <code>password</code>\n </td>\n <td>\n A\n <a href=\"/en-US/docs/Web/JavaScript/Guide/Regular_expressions\">JavaScript regular expression</a>\n (compiled with the <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/global\"><code>global</code></a>, <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/ignoreCase\"><code>ignoreCase</code></a>, and\n <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/multiline\"><code>multiline</code></a> flags <em>disabled</em>)\n </td>\n <td>The value must match the pattern.</td>\n <td>\n <a href=\"/en-US/docs/Web/API/ValidityState/patternMismatch\"><strong><code>patternMismatch</code></strong></a>\n constraint violation\n </td>\n </tr>\n <tr>\n <td rowspan=\"3\"><code><a href=\"/en-US/docs/Web/HTML/Attributes/min\">min</a></code></td>\n <td><code>range</code>, <code>number</code></td>\n <td>A valid number</td>\n <td rowspan=\"3\">The value must be greater than or equal to the value.</td>\n <td rowspan=\"3\">\n <strong><code><a href=\"/en-US/docs/Web/API/ValidityState/rangeUnderflow\">rangeUnderflow</a></code></strong>\n constraint violation\n </td>\n </tr>\n <tr>\n <td><code>date</code>, <code>month</code>, <code>week</code></td>\n <td>A valid date</td>\n </tr>\n <tr>\n <td><code>datetime-local</code>, <code>time</code></td>\n <td>A valid date and time</td>\n </tr>\n <tr>\n <td rowspan=\"3\"><code><a href=\"/en-US/docs/Web/HTML/Attributes/max\">max</a></code></td>\n <td><code>range</code>, <code>number</code></td>\n <td>A valid number</td>\n <td rowspan=\"3\">The value must be less than or equal to the value</td>\n <td rowspan=\"3\">\n <strong><code><a href=\"/en-US/docs/Web/API/ValidityState/rangeOverflow\">rangeOverflow</a></code></strong>\n constraint violation\n </td>\n </tr>\n <tr>\n <td><code>date</code>, <code>month</code>, <code>week</code></td>\n <td>A valid date</td>\n </tr>\n <tr>\n <td><code>datetime-local</code>, <code>time</code></td>\n <td>A valid date and time</td>\n </tr>\n <tr>\n <td><code><a href=\"/en-US/docs/Web/HTML/Attributes/required\">required</a></code></td>\n <td>\n <code>text</code>, <code>search</code>, <code>url</code>,\n <code>tel</code>, <code>email</code>, <code>password</code>,\n <code>date</code>, <code>datetime-local</code>,\n <code>month</code>, <code>week</code>, <code>time</code>,\n <code>number</code>, <code>checkbox</code>, <code>radio</code>,\n <code>file</code>; also on the <a href=\"/en-US/docs/Web/HTML/Element/select\"><code><select></code></a> and\n <a href=\"/en-US/docs/Web/HTML/Element/textarea\"><code><textarea></code></a> elements\n </td>\n <td>\n <em>none</em> as it is a Boolean attribute: its presence means\n <em>true</em>, its absence means <em>false</em>\n </td>\n <td>There must be a value (if set).</td>\n <td>\n <strong><code><a href=\"/en-US/docs/Web/API/ValidityState/valueMissing\">valueMissing</a></code></strong>\n constraint violation\n </td>\n </tr>\n <tr>\n <td rowspan=\"5\"><code><a href=\"/en-US/docs/Web/HTML/Attributes/step\">step</a></code></td>\n <td><code>date</code></td>\n <td>An integer number of days</td>\n <td rowspan=\"5\">\n Unless the step is set to the <code>any</code> literal, the value must\n be <strong>min</strong> + an integral multiple of the step.\n </td>\n <td rowspan=\"5\">\n <strong><code><a href=\"/en-US/docs/Web/API/ValidityState/stepMismatch\">stepMismatch</a></code></strong>\n constraint violation\n </td>\n </tr>\n <tr>\n <td><code>month</code></td>\n <td>An integer number of months</td>\n </tr>\n <tr>\n <td><code>week</code></td>\n <td>An integer number of weeks</td>\n </tr>\n <tr>\n <td><code>datetime-local</code>, <code>time</code></td>\n <td>An integer number of seconds</td>\n </tr>\n <tr>\n <td><code>range</code>, <code>number</code></td>\n <td>An integer</td>\n </tr>\n <tr>\n <td><code><a href=\"/en-US/docs/Web/HTML/Attributes/minlength\">minlength</a></code></td>\n <td>\n <code>text</code>, <code>search</code>, <code>url</code>,\n <code>tel</code>, <code>email</code>, <code>password</code>; also on the\n <a href=\"/en-US/docs/Web/HTML/Element/textarea\"><code><textarea></code></a> element\n </td>\n <td>An integer length</td>\n <td>\n The number of characters (code points) must not be less than the value\n of the attribute, if non-empty. All newlines are normalized to a single\n character (as opposed to CRLF pairs) for\n <a href=\"/en-US/docs/Web/HTML/Element/textarea\"><code><textarea></code></a>.\n </td>\n <td>\n <strong><code><a href=\"/en-US/docs/Web/API/ValidityState/tooShort\">tooShort</a></code></strong>\n constraint violation\n </td>\n </tr>\n <tr>\n <td><code><a href=\"/en-US/docs/Web/HTML/Attributes/maxlength\">maxlength</a></code></td>\n <td>\n <code>text</code>, <code>search</code>, <code>url</code>,\n <code>tel</code>, <code>email</code>, <code>password</code>; also on the\n <a href=\"/en-US/docs/Web/HTML/Element/textarea\"><code><textarea></code></a> element\n </td>\n <td>An integer length</td>\n <td>\n The number of characters (code points) must not exceed the value of the\n attribute.\n </td>\n <td>\n <strong><code><a href=\"/en-US/docs/Web/API/ValidityState/tooLong\">tooLong</a></code></strong>\n constraint violation\n </td>\n </tr>\n </tbody>\n</table></figure>"}},{"type":"prose","value":{"id":"constraint_validation_process","title":"Constraint validation process","isH3":false,"content":"<p>Constraint validation is done through the Constraint Validation API either on a single form element or at the form level, on the <a href=\"/en-US/docs/Web/HTML/Element/form\"><code><form></code></a> element itself. The constraint validation is done in the following ways:</p>\n<ul>\n <li>By a call to the <code>checkValidity()</code> or <code>reportValidity()</code> method of a form-associated DOM interface, (<a href=\"/en-US/docs/Web/API/HTMLInputElement\"><code>HTMLInputElement</code></a>, <a href=\"/en-US/docs/Web/API/HTMLSelectElement\"><code>HTMLSelectElement</code></a>, <a href=\"/en-US/docs/Web/API/HTMLButtonElement\"><code>HTMLButtonElement</code></a>, <a href=\"/en-US/docs/Web/API/HTMLOutputElement\"><code>HTMLOutputElement</code></a> or <a href=\"/en-US/docs/Web/API/HTMLTextAreaElement\"><code>HTMLTextAreaElement</code></a>), which evaluates the constraints only on this element, allowing a script to get this information. The <code>checkValidity()</code> method returns a Boolean indicating whether the element's value passes its constraints. (This is typically done by the user-agent when determining which of the CSS pseudo-classes, <a href=\"/en-US/docs/Web/CSS/:valid\"><code>:valid</code></a> or <a href=\"/en-US/docs/Web/CSS/:invalid\"><code>:invalid</code></a>, applies.) In contrast, the <code>reportValidity()</code> method reports any constraint failures to the user.</li>\n <li>By a call to the <code>checkValidity()</code> or <code>reportValidity()</code> method on the <a href=\"/en-US/docs/Web/API/HTMLFormElement\"><code>HTMLFormElement</code></a> interface.</li>\n <li>By submitting the form itself.</li>\n</ul>\n<p>Calling <code>checkValidity()</code> is called <em>statically</em> validating the constraints, while calling <code>reportValidity()</code> or submitting the form is called <em>interactively</em> validating the constraints.</p>\n<div class=\"notecard note\">\n <p><strong>Note:</strong></p>\n <ul>\n <li>If the <a href=\"/en-US/docs/Web/HTML/Element/form#novalidate\"><code>novalidate</code></a> attribute is set on the <a href=\"/en-US/docs/Web/HTML/Element/form\"><code><form></code></a> element, interactive validation of the constraints doesn't happen.</li>\n <li>Calling the <code>submit()</code> method on the <a href=\"/en-US/docs/Web/API/HTMLFormElement\"><code>HTMLFormElement</code></a> interface doesn't trigger a constraint validation. In other words, this method sends the form data to the server even if it doesn't satisfy the constraints. Call the <code>click()</code> method on a submit button instead.</li>\n <li>The <code>minlength</code> and <code>maxlength</code> constraints are only checked on user-provided input. They are not checked if a value is set programmatically, even when explicitly calling <code>checkValidity()</code> or <code>reportValidity()</code>.</li>\n </ul>\n</div>"}},{"type":"prose","value":{"id":"complex_constraints_using_the_constraint_validation_api","title":"Complex constraints using the Constraint Validation API","isH3":false,"content":"<p>Using JavaScript and the Constraint API, it is possible to implement more complex constraints, for example, constraints combining several fields, or constraints involving complex calculations.</p>\n<p>Basically, the idea is to trigger JavaScript on some form field event (like <strong>onchange</strong>) to calculate whether the constraint is violated, and then to use the method <code>field.setCustomValidity()</code> to set the result of the validation: an empty string means the constraint is satisfied, and any other string means there is an error and this string is the error message to display to the user.</p>"}},{"type":"prose","value":{"id":"constraint_combining_several_fields_postal_code_validation","title":"Constraint combining several fields: Postal code validation","isH3":true,"content":"<p>The postal code format varies from one country to another. Not only do most countries allow an optional prefix with the country code (like <code>D-</code> in Germany, <code>F-</code> in France or Switzerland), but some countries have postal codes with only a fixed number of digits; others, like the UK, have more complex structures, allowing letters at some specific positions.</p>\n<div class=\"notecard note\">\n <p><strong>Note:</strong> This is not a comprehensive postal code validation library, but rather a demonstration of the key concepts.</p>\n</div>\n<p>As an example, we will add a script checking the constraint validation for a form:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">html</span></div><pre class=\"brush: html notranslate\"><code><form>\n <label for=\"postal-code\">Postal Code: </label>\n <input type=\"text\" id=\"postal-code\" />\n <label for=\"country\">Country: </label>\n <select id=\"country\">\n <option value=\"ch\">Switzerland</option>\n <option value=\"fr\">France</option>\n <option value=\"de\">Germany</option>\n <option value=\"nl\">The Netherlands</option>\n </select>\n <input type=\"submit\" value=\"Validate\" />\n</form>\n</code></pre></div>\n<p>This displays the following form:</p><div class=\"code-example\"><div class=\"example-header\"></div><iframe class=\"sample-code-frame\" title=\"Constraint combining several fields Postal code validation sample\" id=\"frame_constraint_combining_several_fields_postal_code_validation\" src=\"about:blank\" data-live-path=\"/en-US/docs/Web/HTML/Constraint_validation\" data-live-id=\"constraint_combining_several_fields_postal_code_validation\" sandbox=\"allow-same-origin allow-scripts\" loading=\"lazy\"></iframe></div>\n<p>First, we write a function checking the constraint itself:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>function checkPostalCode() {\n // For each country, defines the pattern that the postal code has to follow\n const constraints = {\n ch: [\n \"^(CH-)?\\\\d{4}$\",\n \"Swiss postal codes must have exactly 4 digits: e.g. CH-1950 or 1950\",\n ],\n fr: [\n \"^(F-)?\\\\d{5}$\",\n \"French postal codes must have exactly 5 digits: e.g. F-75012 or 75012\",\n ],\n de: [\n \"^(D-)?\\\\d{5}$\",\n \"German postal codes must have exactly 5 digits: e.g. D-12345 or 12345\",\n ],\n nl: [\n \"^(NL-)?\\\\d{4}\\\\s*([A-RT-Z][A-Z]|S[BCE-RT-Z])$\",\n \"Dutch postal codes must have exactly 4 digits, followed by 2 letters except SA, SD and SS\",\n ],\n };\n\n // Read the country id\n const country = document.getElementById(\"country\").value;\n\n // Get the NPA field\n const postalCodeField = document.getElementById(\"postal-code\");\n\n // Build the constraint checker\n const constraint = new RegExp(constraints[country][0], \"\");\n console.log(constraint);\n\n // Check it!\n if (constraint.test(postalCodeField.value)) {\n // The postal code follows the constraint, we use the ConstraintAPI to tell it\n postalCodeField.setCustomValidity(\"\");\n } else {\n // The postal code doesn't follow the constraint, we use the ConstraintAPI to\n // give a message about the format required for this country\n postalCodeField.setCustomValidity(constraints[country][1]);\n }\n}\n</code></pre></div>\n<p>Then we link it to the <strong>onchange</strong> event for the <a href=\"/en-US/docs/Web/HTML/Element/select\"><code><select></code></a> and the <strong>oninput</strong> event for the <a href=\"/en-US/docs/Web/HTML/Element/input\"><code><input></code></a>:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>window.onload = () => {\n document.getElementById(\"country\").onchange = checkPostalCode;\n document.getElementById(\"postal-code\").oninput = checkPostalCode;\n};\n</code></pre></div>"}},{"type":"prose","value":{"id":"limiting_the_size_of_a_file_before_its_upload","title":"Limiting the size of a file before its upload","isH3":true,"content":"<p>Another common constraint is to limit the size of a file to be uploaded. Checking this on the client side before the file is transmitted to the server requires combining the Constraint Validation API, and especially the <code>field.setCustomValidity()</code> method, with another JavaScript API, here the File API.</p>\n<p>Here is the HTML part:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">html</span></div><pre class=\"brush: html notranslate\"><code><label for=\"fs\">Select a file smaller than 75 kB: </label>\n<input type=\"file\" id=\"fs\" />\n</code></pre></div>\n<p>This displays:</p><div class=\"code-example\"><div class=\"example-header\"></div><iframe class=\"sample-code-frame\" title=\"Limiting the size of a file before its upload sample\" id=\"frame_limiting_the_size_of_a_file_before_its_upload\" src=\"about:blank\" data-live-path=\"/en-US/docs/Web/HTML/Constraint_validation\" data-live-id=\"limiting_the_size_of_a_file_before_its_upload\" sandbox=\"allow-same-origin allow-scripts\" loading=\"lazy\"></iframe></div>\n<p>The JavaScript reads the file selected, uses the <code>File.size()</code> method to get its size, compares it to the (hard coded) limit, and calls the Constraint API to inform the browser if there is a violation:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>function checkFileSize() {\n const fs = document.getElementById(\"fs\");\n const files = fs.files;\n\n // If there is (at least) one file selected\n if (files.length > 0) {\n if (files[0].size > 75 * 1000) {\n // Check the constraint\n fs.setCustomValidity(\"The selected file must not be larger than 75 kB\");\n fs.reportValidity();\n return;\n }\n }\n // No custom constraint violation\n fs.setCustomValidity(\"\");\n}\n</code></pre></div>\n<p>Finally, we hook the method with the correct event:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>window.onload = () => {\n document.getElementById(\"fs\").onchange = checkFileSize;\n};\n</code></pre></div>"}},{"type":"prose","value":{"id":"visual_styling_of_constraint_validation","title":"Visual styling of constraint validation","isH3":false,"content":"<p>Apart from setting constraints, web developers want to control what messages are displayed to the users and how they are styled.</p>"}},{"type":"prose","value":{"id":"controlling_the_look_of_elements","title":"Controlling the look of elements","isH3":true,"content":"<p>The look of elements can be controlled via CSS pseudo-classes.</p>\n<h4 id=\"required_and_optional_css_pseudo-classes\">:required and :optional CSS pseudo-classes</h4>\n<p>The <a href=\"/en-US/docs/Web/CSS/:required\"><code>:required</code></a> and <a href=\"/en-US/docs/Web/CSS/:optional\"><code>:optional</code></a> <a href=\"/en-US/docs/Web/CSS/Pseudo-classes\">pseudo-classes</a> allow writing selectors that match form elements that have the <a href=\"/en-US/docs/Web/HTML/Element/input#required\"><code>required</code></a> attribute, or that don't have it.</p>\n<h4 id=\"placeholder-shown_css_pseudo-class\">:placeholder-shown CSS pseudo-class</h4>\n<p>See <a href=\"/en-US/docs/Web/CSS/:placeholder-shown\"><code>:placeholder-shown</code></a>.</p>\n<h4 id=\"valid_invalid_css_pseudo-classes\">:valid :invalid CSS pseudo-classes</h4>\n<p>The <a href=\"/en-US/docs/Web/CSS/:valid\"><code>:valid</code></a> and <a href=\"/en-US/docs/Web/CSS/:invalid\"><code>:invalid</code></a> <a href=\"/en-US/docs/Web/CSS/Pseudo-classes\">pseudo-classes</a> are used to represent <input> elements whose content validates and fails to validate respectively according to the input's type setting. These classes allow the user to style valid or invalid form elements to make it easier to identify elements that are either formatted correctly or incorrectly.</p>"}},{"type":"prose","value":{"id":"controlling_the_text_of_constraint_violation","title":"Controlling the text of constraint violation","isH3":true,"content":"<p>The following items can help with controlling the text of a constraint violation:</p>\n<ul>\n <li>\n <p>The <code>setCustomValidity(message)</code> method on the following elements:</p>\n <ul>\n <li><a href=\"/en-US/docs/Web/HTML/Element/fieldset\"><code><fieldset></code></a>. Note: Setting a custom validity message on fieldset elements will not prevent form submission in most browsers.</li>\n <li><a href=\"/en-US/docs/Web/HTML/Element/input\"><code><input></code></a></li>\n <li><a href=\"/en-US/docs/Web/HTML/Element/output\"><code><output></code></a></li>\n <li><a href=\"/en-US/docs/Web/HTML/Element/select\"><code><select></code></a></li>\n <li>Submit buttons (created with either a <a href=\"/en-US/docs/Web/HTML/Element/button\"><code><button></code></a> element with the <code>submit</code> type, or an <code>input</code> element with the <a href=\"/en-US/docs/Web/HTML/Element/input/submit\">submit</a> type. Other types of buttons do not participate in constraint validation.</li>\n <li><a href=\"/en-US/docs/Web/HTML/Element/textarea\"><code><textarea></code></a></li>\n </ul>\n </li>\n <li>\n <p>The <a href=\"/en-US/docs/Web/API/ValidityState\"><code>ValidityState</code></a> interface describes the object returned by the <code>validity</code> property of the element types listed above. It represents various ways that an entered value can be invalid. Together, they help explain why an element's value fails to validate, if it's not valid.</p>\n </li>\n</ul>"}}],"toc":[{"text":"Intrinsic and basic constraints","id":"intrinsic_and_basic_constraints"},{"text":"Constraint validation process","id":"constraint_validation_process"},{"text":"Complex constraints using the Constraint Validation API","id":"complex_constraints_using_the_constraint_validation_api"},{"text":"Visual styling of constraint validation","id":"visual_styling_of_constraint_validation"}],"summary":"The creation of web forms has always been a complex task. While marking up the form itself is easy, checking whether each field has a valid and coherent value is more difficult, and informing the user about the problem may become a headache. HTML5 introduced new mechanisms for forms: it added new semantic types for the <input> element and constraint validation to ease the work of checking the form content on the client side. Basic, usual constraints can be checked, without the need for JavaScript, by setting new attributes; more complex constraints can be tested using the Constraint Validation API.","popularity":0.0059,"modified":"2024-11-21T09:46:52.000Z","other_translations":[{"locale":"de","title":"Einschränkungen bei Validierung","native":"Deutsch"},{"locale":"fr","title":"Validation des contraintes","native":"Français"},{"locale":"ja","title":"制約検証","native":"日本語"},{"locale":"ru","title":"Валидация ограничений","native":"Русский"},{"locale":"zh-CN","title":"约束验证","native":"中文 (简体)"}],"pageType":"guide","source":{"folder":"en-us/web/html/constraint_validation","github_url":"https://github.com/mdn/content/blob/main/files/en-us/web/html/constraint_validation/index.md","last_commit_url":"https://github.com/mdn/content/commit/4d3605197ea5c6407aacca2a80cc27a398f04fc8","filename":"index.md"},"short_title":"Constraint validation","parents":[{"uri":"/en-US/docs/Web","title":"References"},{"uri":"/en-US/docs/Web/HTML","title":"HTML"},{"uri":"/en-US/docs/Web/HTML/Constraint_validation","title":"Constraint validation"}],"pageTitle":"Constraint validation - HTML: HyperText Markup Language | MDN","noIndexing":false}}</script></body></html>