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href="/en-US/docs/Learn_web_development/Getting_started/Your_first_website/What_will_your_website_look_like">What will your website look like?</a></li><li><a href="/en-US/docs/Learn_web_development/Getting_started/Your_first_website/Creating_the_content">HTML: Creating the content</a></li><li><a href="/en-US/docs/Learn_web_development/Getting_started/Your_first_website/Styling_the_content">CSS: Styling the content</a></li><li><a href="/en-US/docs/Learn_web_development/Getting_started/Your_first_website/Adding_interactivity">JavaScript: Adding interactivity</a></li><li><a href="/en-US/docs/Learn_web_development/Getting_started/Your_first_website/Publishing_your_website">Publishing your website</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Getting_started/Web_standards">Web standards</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Getting_started/Web_standards/How_the_web_works">How the web works</a></li><li><a href="/en-US/docs/Learn_web_development/Getting_started/Web_standards/The_web_standards_model">The web standards model</a></li><li><a href="/en-US/docs/Learn_web_development/Getting_started/Web_standards/How_browsers_load_websites">How browsers load websites</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Getting_started/Soft_skills">Soft skills</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Getting_started/Soft_skills/Research_and_learning">Research and learning</a></li><li><a href="/en-US/docs/Learn_web_development/Getting_started/Soft_skills/Collaboration_and_teamwork">Collaboration and teamwork</a></li><li><a href="/en-US/docs/Learn_web_development/Getting_started/Soft_skills/Workflows_and_processes">Workflows and processes</a></li><li><a href="/en-US/docs/Learn_web_development/Getting_started/Soft_skills/Job_interviews">Succeeding in job interviews</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn_web_development/Core">Core modules</a></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Core/Structuring_content">Structuring content with HTML</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Basic_HTML_syntax">Basic HTML syntax</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Webpage_metadata">What's in the head? Webpage metadata</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Headings_and_paragraphs">Headings and paragraphs in HTML</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Emphasis_and_importance">Emphasis and importance</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Lists">Lists</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Structuring_documents">Structuring documents</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Advanced_text_features">Advanced text features</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Creating_links">Creating links</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Marking_up_a_letter">Challenge: Marking up a letter</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Structuring_a_page_of_content">Challenge: Structuring a page of content</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/HTML_images">HTML images</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/HTML_video_and_audio">HTML video and audio</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Mozilla_splash_page">Challenge: Mozilla splash page</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/HTML_table_basics">HTML table basics</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Table_accessibility">HTML table accessibility</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Planet_data_table">Challenge: Structuring a planet data table</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/HTML_forms">Forms and buttons in HTML</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Structuring_content/Debugging_HTML">Debugging HTML</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Core/Styling_basics">CSS styling basics</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/What_is_CSS">What is CSS?</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Getting_started">Getting started with CSS</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Styling_a_bio_page">Challenge: Styling a biography page</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Basic_selectors">Basic CSS selectors</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Attribute_selectors">Attribute selectors</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Pseudo_classes_and_elements">Pseudo-classes and pseudo-elements</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Combinators">Combinators</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Box_model">The box model</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Handling_conflicts">Handling conflicts</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Values_and_units">CSS values and units</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Sizing">Sizing items in CSS</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Backgrounds_and_borders">Backgrounds and borders</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Overflow">Overflowing content</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Images_media_forms">Images, media, and form elements</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Tables">Styling tables</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Debugging_CSS">Debugging CSS</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Fundamental_CSS_comprehension">Challenge: Fundamental CSS comprehension</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Fancy_letterheaded_paper">Challenge: Creating fancy letterheaded paper</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Styling_basics/Cool-looking_box">Challenge: A cool-looking box</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Core/Text_styling">CSS text styling</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Core/Text_styling/Fundamentals">Fundamental text and font styling</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Text_styling/Styling_lists">Styling lists</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Text_styling/Styling_links">Styling links</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Text_styling/Web_fonts">Web fonts</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Text_styling/Typesetting_a_homepage">Challenge: Typesetting a community school homepage</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Core/CSS_layout">CSS layout</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Core/CSS_layout/Introduction">Introduction to CSS layout</a></li><li><a href="/en-US/docs/Learn_web_development/Core/CSS_layout/Floats">Floats</a></li><li><a href="/en-US/docs/Learn_web_development/Core/CSS_layout/Positioning">Positioning</a></li><li><a href="/en-US/docs/Learn_web_development/Core/CSS_layout/Flexbox">Flexbox</a></li><li><a href="/en-US/docs/Learn_web_development/Core/CSS_layout/Grids">CSS grid layout</a></li><li><a href="/en-US/docs/Learn_web_development/Core/CSS_layout/Responsive_Design">Responsive design</a></li><li><a href="/en-US/docs/Learn_web_development/Core/CSS_layout/Media_queries">Media query fundamentals</a></li><li><a href="/en-US/docs/Learn_web_development/Core/CSS_layout/Fundamental_Layout_Comprehension">Challenge: Fundamental layout comprehension</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Core/Scripting">Dynamic scripting with JavaScript</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/What_is_JavaScript">What is JavaScript?</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/A_first_splash">A first splash into JavaScript</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/What_went_wrong">What went wrong? Troubleshooting JavaScript</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Variables">Storing the information you need — Variables</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Math">Basic math in JavaScript — numbers and operators</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Strings">Handling text — strings in JavaScript</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Useful_string_methods">Useful string methods</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Arrays">Arrays</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Silly_story_generator">Challenge: Silly story generator</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Conditionals">Making decisions in your code — conditionals</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Loops">Looping code</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Functions">Functions — reusable blocks of code</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Build_your_own_function">Build your own function</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Return_values">Function return values</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Events">Introduction to events</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Event_bubbling">Event bubbling</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Image_gallery">Challenge: Image gallery</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Object_basics">JavaScript object basics</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/DOM_scripting">DOM scripting introduction</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Network_requests">Making network requests with JavaScript</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/JSON">Working with JSON</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Scripting/Debugging_JavaScript">Debugging JavaScript and handling errors</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries">JavaScript frameworks and libraries</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/Introduction">Introduction to client-side frameworks</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/Main_features">Framework main features</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_getting_started">Getting started with React</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_todo_list_beginning">Beginning our React todo list</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_components">Componentizing our React app</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_interactivity_events_state">React interactivity: Events and state</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_interactivity_filtering_conditional_rendering">React interactivity: Editing, filtering, conditional rendering</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_accessibility">Accessibility in React</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_resources">React resources</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Core/Accessibility">Accessibility</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Core/Accessibility/What_is_accessibility">What is accessibility?</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Accessibility/Tooling">Accessibility tooling and assistive technology</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Accessibility/HTML">HTML: A good basis for accessibility</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Accessibility/CSS_and_JavaScript">CSS and JavaScript accessibility best practices</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Accessibility/WAI-ARIA_basics">WAI-ARIA basics</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Accessibility/Multimedia">Accessible multimedia</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Accessibility/Mobile">Mobile accessibility</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Accessibility/Accessibility_troubleshooting">Challenge: Accessibility troubleshooting</a></li></ol></details></li><li><a href="/en-US/docs/Learn_web_development/Core/Design_for_developers">Design for developers</a></li><li><a href="/en-US/docs/Learn_web_development/Core/Version_control">Version control</a></li><li class="section"><a href="/en-US/docs/Learn_web_development/Extensions">Extension modules</a></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects">Advanced JavaScript objects</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Object_prototypes">Object prototypes</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Object-oriented_programming">Object-oriented programming</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Classes_in_JavaScript">Classes in JavaScript</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Object_building_practice">Object building practice</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Adding_bouncing_balls_features">Challenge: Adding features to our bouncing balls demo</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Extensions/Client-side_APIs">Client-side web APIs</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Extensions/Client-side_APIs/Introduction">Introduction to web APIs</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Client-side_APIs/Video_and_audio_APIs">Video and Audio APIs</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Client-side_APIs/Drawing_graphics">Drawing graphics</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Client-side_APIs/Client-side_storage">Client-side storage</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Client-side_APIs/Third_party_APIs">Third-party APIs</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Extensions/Async_JS">Asynchronous JavaScript</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Extensions/Async_JS/Introducing">Introducing asynchronous JavaScript</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Async_JS/Promises">How to use promises</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Async_JS/Implementing_a_promise-based_API">How to implement a promise-based API</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Async_JS/Introducing_workers">Introducing workers</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Async_JS/Sequencing_animations">Challenge: Sequencing animations</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Extensions/Forms">Web forms</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Extensions/Forms/Your_first_form">Your first form</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Forms/How_to_structure_a_web_form">How to structure a web form</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Forms/Basic_native_form_controls">Basic native form controls</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Forms/HTML5_input_types">The HTML5 input types</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Forms/Other_form_controls">Other form controls</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Forms/Styling_web_forms">Styling web forms</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Forms/Advanced_form_styling">Advanced form styling</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Forms/UI_pseudo-classes">UI pseudo-classes</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Forms/Form_validation">Client-side form validation</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Forms/Sending_and_retrieving_form_data">Sending form data</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Extensions/Client-side_tools">Understanding client-side tools</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Extensions/Client-side_tools/Overview">Client-side tooling overview</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Client-side_tools/Package_management">Package management basics</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Client-side_tools/Introducing_complete_toolchain">Introducing a complete toolchain</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Client-side_tools/Deployment">Deploying our app</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Extensions/Server-side">Server-side websites</a></summary><ol><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/First_steps">Server-side first steps</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/First_steps/Introduction">Introduction to the server side</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/First_steps/Client-Server_overview">Client-Server Overview</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/First_steps/Web_frameworks">Server-side web frameworks</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/First_steps/Website_security">Website security</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django">Django web framework (Python)</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Introduction">Django introduction</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django/development_environment">Setting up a Django development environment</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Tutorial_local_library_website">Django Tutorial: The Local Library website</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django/skeleton_website">Django Tutorial Part 2: Creating a skeleton website</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Models">Django Tutorial Part 3: Using models</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Admin_site">Django Tutorial Part 4: Django admin site</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Home_page">Django Tutorial Part 5: Creating our home page</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Generic_views">Django Tutorial Part 6: Generic list and detail views</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Sessions">Django Tutorial Part 7: Sessions framework</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Authentication">Django Tutorial Part 8: User authentication and permissions</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Forms">Django Tutorial Part 9: Working with forms</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Testing">Django Tutorial Part 10: Testing a Django web application</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Deployment">Django Tutorial Part 11: Deploying Django to production</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django/web_application_security">Django web application security</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Django/django_assessment_blog">Assessment: DIY Django mini blog</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs">Express web framework (Node.js)</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/Introduction">Express/Node introduction</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/development_environment">Setting up a Node development environment</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/Tutorial_local_library_website">Express Tutorial: The Local Library website</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/skeleton_website">Express Tutorial Part 2: Creating a skeleton website</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/mongoose">Express Tutorial Part 3: Using a Database (with Mongoose)</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/routes">Express Tutorial Part 4: Routes and controllers</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/Displaying_data">Express Tutorial Part 5: Displaying library data</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/forms">Express Tutorial Part 6: Working with forms</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/deployment">Express Tutorial Part 7: Deploying to production</a></li></ol></details></li></ol></details></li><li class="toggle"><details open=""><summary><a href="/en-US/docs/Learn_web_development/Extensions/Performance">Web performance</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Extensions/Performance/why_web_performance">The "why" of web performance</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Performance/What_is_web_performance">What is web performance?</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Performance/Perceived_performance">Perceived performance</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Performance/Measuring_performance">Measuring performance</a></li><li><em><a href="/en-US/docs/Learn_web_development/Extensions/Performance/Multimedia" aria-current="page">Multimedia: Images</a></em></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Performance/video">Multimedia: video</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Performance/JavaScript">JavaScript performance optimization</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Performance/HTML">HTML performance optimization</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Performance/CSS">CSS performance optimization</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Performance/business_case_for_performance">The business case for web performance</a></li></ol></details></li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Extensions/Testing">Testing</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Extensions/Testing/Introduction">Introduction to cross-browser testing</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Testing/Testing_strategies">Strategies for carrying out testing</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Testing/HTML_and_CSS">Handling common HTML and CSS problems</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Testing/Feature_detection">Implementing feature detection</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Testing/Automated_testing">Introduction to automated testing</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Testing/Your_own_automation_environment">Setting up your own test automation environment</a></li></ol></details></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Transform_animate">Transform and animate CSS</a></li><li><a href="/en-US/docs/Learn_web_development/Extensions/Security_privacy">Security and privacy</a></li><li class="section">Further resources</li><li class="toggle"><details><summary><a href="/en-US/docs/Learn_web_development/Howto">How to solve common problems</a></summary><ol><li><a href="/en-US/docs/Learn_web_development/Howto/Solve_HTML_problems">Solve common HTML problems</a></li><li><a href="/en-US/docs/Learn_web_development/Howto/Solve_CSS_problems">Solve common CSS problems</a></li><li><a href="/en-US/docs/Learn_web_development/Howto/Solve_JavaScript_problems">Solve common JavaScript problems</a></li><li><a href="/en-US/docs/Learn_web_development/Howto/Web_mechanics">Web mechanics</a></li><li><a href="/en-US/docs/Learn_web_development/Howto/Tools_and_setup">Tools and setup</a></li><li><a href="/en-US/docs/Learn_web_development/Howto/Design_and_accessibility">Design and accessibility</a></li></ol></details></li><li><a href="/en-US/docs/Learn_web_development/About">About</a></li><li><a href="/en-US/docs/Learn_web_development/Educators">Resources for educators</a></li><li><a href="/en-US/docs/Learn_web_development/Changelog">Changelog</a></li></ol></div></div><section class="place side"></section></nav></aside><div class="toc-container"><aside class="toc"><nav><div class="document-toc-container"><section class="document-toc"><header><h2 class="document-toc-heading">In this article</h2></header><ul class="document-toc-list"><li class="document-toc-item "><a class="document-toc-link" href="#why_optimize_your_multimedia">Why optimize your multimedia?</a></li><li class="document-toc-item "><a class="document-toc-link" href="#optimizing_image_delivery">Optimizing image delivery</a></li><li class="document-toc-item "><a class="document-toc-link" href="#conclusion">Conclusion</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>Multimedia: Images</h1></header><div class="section-content"><ul class="prev-next"><li><a class="button secondary" href="/en-US/docs/Learn_web_development/Extensions/Performance/Measuring_performance"><span class="button-wrap"> Previous </span></a></li><li><a class="button secondary" href="/en-US/docs/Learn_web_development/Extensions/Performance"><span class="button-wrap"> Overview: Web performance</span></a></li><li><a class="button secondary" href="/en-US/docs/Learn_web_development/Extensions/Performance/video"><span class="button-wrap"> Next </span></a></li></ul> <p>Media, namely images and video, account for over 70% of the bytes downloaded for the average website. In terms of download performance, eliminating media, and reducing file size is the low-hanging fruit. This article looks at optimizing images and video to improve web performance.</p> <figure class="table-container"><table> <tbody> <tr> <th scope="row">Prerequisites:</th> <td> <a href="/en-US/docs/Learn_web_development/Getting_started/Environment_setup/Installing_software">Basic software installed</a>, and basic knowledge of <a href="/en-US/docs/Learn_web_development/Getting_started/Your_first_website">client-side web technologies</a>. </td> </tr> <tr> <th scope="row">Objective:</th> <td> To learn about the various image formats, their impact on performance, and how to reduce the impact of images on overall page load time. </td> </tr> </tbody> </table></figure> <div class="notecard note"> <p><strong>Note:</strong> This is a high-level introduction to optimizing multimedia delivery on the web, covering general principles and techniques. For a more in-depth guide, see <a href="https://web.dev/learn/images" class="external" target="_blank">https://web.dev/learn/images</a>.</p> </div></div><section aria-labelledby="why_optimize_your_multimedia"><h2 id="why_optimize_your_multimedia"><a href="#why_optimize_your_multimedia">Why optimize your multimedia?</a></h2><div class="section-content"><p>For the average website, <a href="https://discuss.httparchive.org/t/state-of-the-web-top-image-optimization-strategies/1367" class="external" target="_blank">51% of its bandwidth comes from imagery, followed by video at 25%</a>, so it's safe to say it's important to address and optimize your multimedia content.</p> <p>You need to be considerate of data usage. Many people are on capped data plans or even pay-as-you-go where they are literally paying by the megabyte. This isn't an emerging market problem either. As of 2018, 24% of the United Kingdom still use pay-as-you-go according to <a href="https://www.ofcom.org.uk/siteassets/resources/documents/research-and-data/technology-research/technology-tracker/technology-tracker-h1-2018-data-tables?v=323142" class="external" target="_blank">OFCOM Nations & regions technology tracker - H1 2018 (PDF)</a>.</p> <p>You also need to be considerate of memory as many mobile devices have limited RAM. It's important to remember that when images are downloaded, they need to be stored in memory.</p></div></section><section aria-labelledby="optimizing_image_delivery"><h2 id="optimizing_image_delivery"><a href="#optimizing_image_delivery">Optimizing image delivery</a></h2><div class="section-content"><p>Despite being the largest consumer of bandwidth, the impact of image downloading on <a href="/en-US/docs/Learn_web_development/Extensions/Performance/Perceived_performance">perceived performance</a> is far lower than many expect (primarily because the page text content is downloaded immediately and users can see the images being rendered as they arrive). However, for a good user experience it's still important that a visitor can see them as soon as possible.</p></div></section><section aria-labelledby="loading_strategy"><h3 id="loading_strategy"><a href="#loading_strategy">Loading strategy</a></h3><div class="section-content"><p>One of the biggest improvements to most websites is <a href="/en-US/docs/Web/Performance/Lazy_loading">lazy-loading</a> images beneath the fold, rather than downloading them all on initial page load regardless of whether a visitor scrolls to see them or not. Browsers provide this natively via the <a href="/en-US/docs/Web/HTML/Element/img#loading"><code>loading="lazy"</code></a> attribute on the <code><img></code> element, and there are also many client-side JavaScript libraries that can do this.</p> <p>Beyond loading a subset of images, you should look into the format of the images themselves:</p> <ul> <li>Are you loading the most optimal file formats?</li> <li>Have you compressed the images well?</li> <li>Are you loading the correct sizes?</li> </ul> <h4 id="the_most_optimal_format">The most optimal format</h4> <p>The optimal file format typically depends on the character of the image.</p> <div class="notecard note"> <p><strong>Note:</strong> For general information on image types see the <a href="/en-US/docs/Web/Media/Guides/Formats/Image_types">Image file type and format guide</a></p> </div> <p>The <a href="/en-US/docs/Web/Media/Guides/Formats/Image_types#svg_scalable_vector_graphics">SVG</a> format is more appropriate for images that have few colors and that are not photo-realistic. This requires the source to be available as in a vector graphic format. Should such an image only exist as a bitmap, then <a href="/en-US/docs/Web/Media/Guides/Formats/Image_types#png_portable_network_graphics">PNG</a> would be the fallback format to choose. Examples of these types of motifs are logos, illustrations, charts, or icons (note: SVGs are far better than icon fonts!). Both formats support transparency.</p> <p>PNGs can be saved with three different output combinations:</p> <ul> <li>24-bit color + 8-bit transparency — offer full-color accuracy and smooth transparency at the expense of size. You probably want to avoid this combination in favor of WebP (see below).</li> <li>8-bit color + 8-bit transparency — offer no more than 255 colors but maintain smooth transparencies. The size is not too big. Those are the PNGs you would probably want.</li> <li>8-bit color + 1-bit transparency — offer no more than 255 colors and just offer no or full transparency per pixel which makes the transparency borders appear hard and jagged. The size is small but the price is visual fidelity.</li> </ul> <p>A good online tool for optimizing SVGs is <a href="https://jakearchibald.github.io/svgomg/" class="external" target="_blank">SVGOMG</a>. For PNGs there is <a href="https://imageoptim.com/online" class="external" target="_blank">ImageOptim online</a> or <a href="https://squoosh.app/" class="external" target="_blank">Squoosh</a>.</p> <p>With photographic motifs that do not feature transparency, there is a much wider range of formats to choose from. If you want to play it safe, then you would go for well-compressed <strong>Progressive JPEGs</strong>. Progressive JPEGs, in contrast to normal JPEGs, render progressively (hence the name), meaning the user sees a low-resolution version that gains clarity as the image downloads, rather than the image loading at full resolution top-to-bottom or even only rendering once completely downloaded. A good compressor for these would be MozJPEG, e.g. available to use in the online image optimization tool <a href="https://squoosh.app/" class="external" target="_blank">Squoosh</a>. A quality setting of 75% should yield decent results.</p> <p>Other formats improve on JPEG's capabilities regarding compression, but are not available on every browser:</p> <ul> <li> <p><a href="/en-US/docs/Web/Media/Guides/Formats/Image_types#webp_image">WebP</a> — Excellent choice for both images and animated images. WebP offers much better compression than PNG or JPEG with support for higher color depths, animated frames, transparency, etc. (but not progressive display.). Supported by all major browsers except Safari 14 on macOS desktop Big Sur or earlier.</p> <div class="notecard note"> <p><strong>Note:</strong> Despite Apple <a href="https://developer.apple.com/videos/play/wwdc2020/10663/?time=1174" class="external" target="_blank">announcing support for WebP in Safari 14</a>, Safari versions earlier than 16.0 don't display <code>.webp</code> images successfully on macOS desktop versions earlier than 11/Big Sur. Safari for iOS 14 <em>does</em> display <code>.webp</code> images successfully.</p> </div> </li> <li> <p><a href="/en-US/docs/Web/Media/Guides/Formats/Image_types#avif_image">AVIF</a> — Good choice for both images and animated images due to high performance and royalty-free image format (even more efficient than WebP, but not as widely supported). It is now supported on Chrome, Edge, Opera, and Firefox. <a href="https://squoosh.app/" class="external" target="_blank">Squoosh</a> is a good online tool for converting previous image formats to AVIF.</p> </li> <li> <p><strong>JPEG2000</strong> — once to be the successor to JPEG but only supported in Safari. Doesn't support progressive display either.</p> </li> </ul> <p>Given the narrow support for JPEG-XR and JPEG2000, and also taking decode costs into the equation, the only serious contender for JPEG is WebP. Which is why you could offer your images in that flavor too. This can be done via the <code><picture></code> element with the help of a <code><source></code> element equipped with a <a href="/en-US/docs/Web/HTML/Element/picture#the_type_attribute">type attribute</a>.</p> <p>If all of this sounds a bit complicated or feels like too much work for your team then there are also online services that you can use as image CDNs that will automate the serving of the correct image format on the fly, according to the type of device or browser requesting the image. Popular choices include <a href="https://cloudinary.com/blog/make_all_images_on_your_website_responsive_in_3_easy_steps" class="external" target="_blank">Cloudinary</a>, <a href="https://imageengine.io/" class="external" target="_blank">Image Engine</a>, <a href="https://imagekit.io/docs/image-optimization#automatic-format-conversion" class="external" target="_blank">ImageKit</a>, and <a href="https://www.imgix.com/" class="external" target="_blank">imgix</a>.</p> <p>Finally, should you want to include animated images on your page, then know that Safari allows the use of video files within <code><img></code> and <code><picture></code> elements. These also allow you to add in an <strong>Animated WebP</strong> for all other modern browsers.</p> <div class="code-example"><div class="example-header"><span class="language-name">html</span></div><pre class="brush: html notranslate"><code><picture> <source type="video/mp4" src="giphy.mp4" /> <source type="image/webp" src="giphy.webp" /> <img src="giphy.gif" alt="A GIF animation" /> </picture> </code></pre></div> <h4 id="serving_the_optimal_size">Serving the optimal size</h4> <p>In image delivery the "one size fits all" approach will not yield the best results, meaning that for smaller screens you would want to serve images with smaller resolution and vice versa for larger screens. On top of that, you'd also want to serve higher resolution images to those devices that boast a high DPI screen (e.g. "Retina"). So apart from creating plenty of intermediate image variants you also need a way to serve the right file to the right browser. That's where you would need to upgrade your <code><picture></code> and <code><source></code> elements with <a href="/en-US/docs/Web/HTML/Element/source#media">media</a> and/or <a href="/en-US/docs/Web/HTML/Element/source#sizes">sizes</a> attributes. A detailed article on how to combine all of these attributes can be found <a href="https://www.smashingmagazine.com/2014/05/responsive-images-done-right-guide-picture-srcset/" class="external" target="_blank">here</a>.</p> <p>Two interesting effects to keep in mind regarding high dpi screens is that:</p> <ul> <li>with a high DPI screen, humans will spot compression artifacts a lot later, meaning that for images meant for these screens you can crank up compression beyond usual.</li> <li><a href="https://observablehq.com/@eeeps/visual-acuity-and-device-pixel-ratio" class="external" target="_blank">Only a very few people can spot an increase in resolution beyond 2× DPI</a>, which means you don't need to serve images resolving higher than 2×.</li> </ul> <h4 id="controlling_the_priority_and_ordering_of_downloading_images">Controlling the priority (and ordering) of downloading images</h4> <p>Getting the most important images in front of visitors sooner than the less important can deliver improved perceived performance.</p> <p>The first thing to check is that your content images use <code><img></code> or <code><picture></code> elements and your background images are defined in CSS with <code>background-image</code> — images referenced in <code><img></code> elements are assigned a higher loading priority than background images.</p> <p>Secondly, with the adoption of Priority Hints, you can control the priority further by adding a <code>fetchPriority</code> attribute to your image tags. An example use case for priority hints on images are carousels where the first image is a higher priority than the subsequent images.</p></div></section><section aria-labelledby="rendering_strategy_preventing_jank_when_loading_images"><h3 id="rendering_strategy_preventing_jank_when_loading_images"><a href="#rendering_strategy_preventing_jank_when_loading_images">Rendering strategy: preventing jank when loading images</a></h3><div class="section-content"><p>As images are loaded asynchronously and continue to load after the first paint, if their dimensions aren't defined before load, they can cause reflows to the page content. For example, when text gets pushed down the page by images loading. For this reason, it's important to set <code>width</code> and <code>height</code> attributes so that the browser can reserve space for them in the layout.</p> <p>When the <code>width</code> and <code>height</code> attributes of an image are included on an HTML <a href="/en-US/docs/Web/HTML/Element/img"><code><img></code></a> element, the <a href="/en-US/docs/Web/CSS/CSS_box_sizing/Understanding_aspect-ratio#adjusting_aspect_ratios_of_replaced_elements">aspect ratio of the image</a> can be calculated by the browser prior to the image being loaded. This <a href="/en-US/docs/Glossary/Aspect_ratio">aspect ratio</a> is used to reserve the space needed to display the image, reducing or even preventing a layout shift when the image is downloaded and painted to the screen. Reducing layout shift is a major component of good user experience and web performance.</p> <p>Browsers begin rendering content as HTML is parsed, often before all assets, including images, are downloaded. Including dimensions enable browsers to reserve a correctly-sized placeholder box for each image to appear in when the images are loaded when first rendering the page.</p> <p><img src="/en-US/docs/Learn_web_development/Extensions/Performance/Multimedia/ar-guide.jpg" alt="Two screenshots the first without an image but with space reserved, the second showing the image loaded into the reserved space." width="1200" height="857" loading="lazy"></p> <p>Without the <code>width</code> and <code>height</code> attributes, no placeholder space is created, creating a noticeable <a href="/en-US/docs/Glossary/Jank">jank</a>, or layout shift, in the page when the image loads after the page is rendered. Page reflow and repaints are performance and usability issues.</p> <p>The <a href="/en-US/docs/Glossary/CLS">CLS</a> metric measures jank on page load, or how much visible content shifts in the viewport and by how much. The main culprits of bad CLS are replaced elements without declared dimensions that reflow when the asset loads, including images, ad, embeds, and iframes without an size or <a href="/en-US/docs/Web/CSS/aspect-ratio"><code>aspect-ratio</code></a> and web fonts.</p> <p>In responsive designs, when a container is narrower than an image, the following CSS is generally used to keep images from breaking out of their containers:</p> <div class="code-example"><div class="example-header"><span class="language-name">css</span></div><pre class="brush: css notranslate"><code>img { max-width: 100%; height: auto; } </code></pre></div> <p>While useful for responsive layouts, this causes jank and poor CLS when width and height information are not included, as if no height information is present when the <code><img></code> element is parsed but before the image has loaded, this CSS effectively has set the height to 0. When the image loads after the page has been initially rendered to the screen, the page reflows and repaints creating a layout shift as it creates space for the newly determined height.</p> <p>Browsers have a mechanism for sizing images before the actual image is loaded. When an <code><img></code>, <code><video></code>, or <code><input type="button"></code> element has <code>width</code> and <code>height</code> attributes set on it, its aspect ratio is calculated before load time, and is available to the browser, using the dimensions provided.</p> <p>The aspect ratio is then used to calculate the height, and therefore, the correct size is applied to the <code><img></code> element, meaning that the aforementioned jank will not occur or be minimal if the listed dimensions are not fully accurate when the image loads.</p> <p>The aspect ratio is used to reserve space only on the image load. Once the image has loaded, the intrinsic aspect ratio of the loaded image or the value of the <code>aspect-ratio</code> property is used rather than the aspect ratio from the attributes. This ensures that it displays at the correct aspect ratio even if the attribute dimensions are not accurate.</p> <p>While developer best practices from the last decade may have recommended omitting the <code>width</code> and <code>height</code> attributes of an image on an HTML <a href="/en-US/docs/Web/HTML/Element/img"><code><img></code></a>, due to aspect ratio mapping, including these two attributes is considered a developer best practice.</p> <p>For any background images, it's important you set a <code>background-color</code> value so any content overlaid is still readable before the image has downloaded.</p></div></section><section aria-labelledby="conclusion"><h2 id="conclusion"><a href="#conclusion">Conclusion</a></h2><div class="section-content"><p>In this section, we took a look at image optimization. You now have a general understanding of how to optimize half of the average website's average bandwidth total. This is just one of the types of media consuming users' bandwidth and slowing down page load. 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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/Learn_web_development/Extensions/Performance/Multimedia","doc":{"body":[{"type":"prose","value":{"id":null,"title":null,"isH3":false,"content":"<ul class=\"prev-next\"><li><a class=\"button secondary\" href=\"/en-US/docs/Learn_web_development/Extensions/Performance/Measuring_performance\"><span class=\"button-wrap\"> Previous </span></a></li><li><a class=\"button secondary\" href=\"/en-US/docs/Learn_web_development/Extensions/Performance\"><span class=\"button-wrap\"> Overview: Web performance</span></a></li><li><a class=\"button secondary\" href=\"/en-US/docs/Learn_web_development/Extensions/Performance/video\"><span class=\"button-wrap\"> Next </span></a></li></ul>\n<p>Media, namely images and video, account for over 70% of the bytes downloaded for the average website. In terms of download performance, eliminating media, and reducing file size is the low-hanging fruit. This article looks at optimizing images and video to improve web performance.</p>\n<figure class=\"table-container\"><table>\n <tbody>\n <tr>\n <th scope=\"row\">Prerequisites:</th>\n <td>\n <a href=\"/en-US/docs/Learn_web_development/Getting_started/Environment_setup/Installing_software\">Basic software installed</a>, and basic knowledge of\n <a href=\"/en-US/docs/Learn_web_development/Getting_started/Your_first_website\">client-side web technologies</a>.\n </td>\n </tr>\n <tr>\n <th scope=\"row\">Objective:</th>\n <td>\n To learn about the various image formats, their impact on performance,\n and how to reduce the impact of images on overall page load time.\n </td>\n </tr>\n </tbody>\n</table></figure>\n<div class=\"notecard note\">\n<p><strong>Note:</strong>\nThis is a high-level introduction to optimizing multimedia delivery on the web, covering general principles and techniques. For a more in-depth guide, see <a href=\"https://web.dev/learn/images\" class=\"external\" target=\"_blank\">https://web.dev/learn/images</a>.</p>\n</div>"}},{"type":"prose","value":{"id":"why_optimize_your_multimedia","title":"Why optimize your multimedia?","isH3":false,"content":"<p>For the average website, <a href=\"https://discuss.httparchive.org/t/state-of-the-web-top-image-optimization-strategies/1367\" class=\"external\" target=\"_blank\">51% of its bandwidth comes from imagery, followed by video at 25%</a>, so it's safe to say it's important to address and optimize your multimedia content.</p>\n<p>You need to be considerate of data usage. Many people are on capped data plans or even pay-as-you-go where they are literally paying by the megabyte. This isn't an emerging market problem either. As of 2018, 24% of the United Kingdom still use pay-as-you-go according to <a href=\"https://www.ofcom.org.uk/siteassets/resources/documents/research-and-data/technology-research/technology-tracker/technology-tracker-h1-2018-data-tables?v=323142\" class=\"external\" target=\"_blank\">OFCOM Nations & regions technology tracker - H1 2018 (PDF)</a>.</p>\n<p>You also need to be considerate of memory as many mobile devices have limited RAM. It's important to remember that when images are downloaded, they need to be stored in memory.</p>"}},{"type":"prose","value":{"id":"optimizing_image_delivery","title":"Optimizing image delivery","isH3":false,"content":"<p>Despite being the largest consumer of bandwidth, the impact of image downloading on <a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/Perceived_performance\">perceived performance</a> is far lower than many expect (primarily because the page text content is downloaded immediately and users can see the images being rendered as they arrive). However, for a good user experience it's still important that a visitor can see them as soon as possible.</p>"}},{"type":"prose","value":{"id":"loading_strategy","title":"Loading strategy","isH3":true,"content":"<p>One of the biggest improvements to most websites is <a href=\"/en-US/docs/Web/Performance/Lazy_loading\">lazy-loading</a> images beneath the fold, rather than downloading them all on initial page load regardless of whether a visitor scrolls to see them or not. Browsers provide this natively via the <a href=\"/en-US/docs/Web/HTML/Element/img#loading\"><code>loading=\"lazy\"</code></a> attribute on the <code><img></code> element, and there are also many client-side JavaScript libraries that can do this.</p>\n<p>Beyond loading a subset of images, you should look into the format of the images themselves:</p>\n<ul>\n<li>Are you loading the most optimal file formats?</li>\n<li>Have you compressed the images well?</li>\n<li>Are you loading the correct sizes?</li>\n</ul>\n<h4 id=\"the_most_optimal_format\">The most optimal format</h4>\n<p>The optimal file format typically depends on the character of the image.</p>\n<div class=\"notecard note\">\n<p><strong>Note:</strong>\nFor general information on image types see the <a href=\"/en-US/docs/Web/Media/Guides/Formats/Image_types\">Image file type and format guide</a></p>\n</div>\n<p>The <a href=\"/en-US/docs/Web/Media/Guides/Formats/Image_types#svg_scalable_vector_graphics\">SVG</a> format is more appropriate for images that have few colors and that are not photo-realistic. This requires the source to be available as in a vector graphic format. Should such an image only exist as a bitmap, then <a href=\"/en-US/docs/Web/Media/Guides/Formats/Image_types#png_portable_network_graphics\">PNG</a> would be the fallback format to choose. Examples of these types of motifs are logos, illustrations, charts, or icons (note: SVGs are far better than icon fonts!). Both formats support transparency.</p>\n<p>PNGs can be saved with three different output combinations:</p>\n<ul>\n<li>24-bit color + 8-bit transparency — offer full-color accuracy and smooth transparency at the expense of size. You probably want to avoid this combination in favor of WebP (see below).</li>\n<li>8-bit color + 8-bit transparency — offer no more than 255 colors but maintain smooth transparencies. The size is not too big. Those are the PNGs you would probably want.</li>\n<li>8-bit color + 1-bit transparency — offer no more than 255 colors and just offer no or full transparency per pixel which makes the transparency borders appear hard and jagged. The size is small but the price is visual fidelity.</li>\n</ul>\n<p>A good online tool for optimizing SVGs is <a href=\"https://jakearchibald.github.io/svgomg/\" class=\"external\" target=\"_blank\">SVGOMG</a>. For PNGs there is <a href=\"https://imageoptim.com/online\" class=\"external\" target=\"_blank\">ImageOptim online</a> or <a href=\"https://squoosh.app/\" class=\"external\" target=\"_blank\">Squoosh</a>.</p>\n<p>With photographic motifs that do not feature transparency, there is a much wider range of formats to choose from. If you want to play it safe, then you would go for well-compressed <strong>Progressive JPEGs</strong>. Progressive JPEGs, in contrast to normal JPEGs, render progressively (hence the name), meaning the user sees a low-resolution version that gains clarity as the image downloads, rather than the image loading at full resolution top-to-bottom or even only rendering once completely downloaded. A good compressor for these would be MozJPEG, e.g. available to use in the online image optimization tool <a href=\"https://squoosh.app/\" class=\"external\" target=\"_blank\">Squoosh</a>. A quality setting of 75% should yield decent results.</p>\n<p>Other formats improve on JPEG's capabilities regarding compression, but are not available on every browser:</p>\n<ul>\n<li>\n<p><a href=\"/en-US/docs/Web/Media/Guides/Formats/Image_types#webp_image\">WebP</a> — Excellent choice for both images and animated images. WebP offers much better compression than PNG or JPEG with support for higher color depths, animated frames, transparency, etc. (but not progressive display.). Supported by all major browsers except Safari 14 on macOS desktop Big Sur or earlier.</p>\n<div class=\"notecard note\">\n<p><strong>Note:</strong>\nDespite Apple <a href=\"https://developer.apple.com/videos/play/wwdc2020/10663/?time=1174\" class=\"external\" target=\"_blank\">announcing support for WebP in Safari 14</a>, Safari versions earlier than 16.0 don't display <code>.webp</code> images successfully on macOS desktop versions earlier than 11/Big Sur. Safari for iOS 14 <em>does</em> display <code>.webp</code> images successfully.</p>\n</div>\n</li>\n<li>\n<p><a href=\"/en-US/docs/Web/Media/Guides/Formats/Image_types#avif_image\">AVIF</a> — Good choice for both images and animated images due to high performance and royalty-free image format (even more efficient than WebP, but not as widely supported). It is now supported on Chrome, Edge, Opera, and Firefox. <a href=\"https://squoosh.app/\" class=\"external\" target=\"_blank\">Squoosh</a> is a good online tool for converting previous image formats to AVIF.</p>\n</li>\n<li>\n<p><strong>JPEG2000</strong> — once to be the successor to JPEG but only supported in Safari. Doesn't support progressive display either.</p>\n</li>\n</ul>\n<p>Given the narrow support for JPEG-XR and JPEG2000, and also taking decode costs into the equation, the only serious contender for JPEG is WebP. Which is why you could offer your images in that flavor too. This can be done via the <code><picture></code> element with the help of a <code><source></code> element equipped with a <a href=\"/en-US/docs/Web/HTML/Element/picture#the_type_attribute\">type attribute</a>.</p>\n<p>If all of this sounds a bit complicated or feels like too much work for your team then there are also online services that you can use as image CDNs that will automate the serving of the correct image format on the fly, according to the type of device or browser requesting the image. Popular choices include <a href=\"https://cloudinary.com/blog/make_all_images_on_your_website_responsive_in_3_easy_steps\" class=\"external\" target=\"_blank\">Cloudinary</a>, <a href=\"https://imageengine.io/\" class=\"external\" target=\"_blank\">Image Engine</a>, <a href=\"https://imagekit.io/docs/image-optimization#automatic-format-conversion\" class=\"external\" target=\"_blank\">ImageKit</a>, and <a href=\"https://www.imgix.com/\" class=\"external\" target=\"_blank\">imgix</a>.</p>\n<p>Finally, should you want to include animated images on your page, then know that Safari allows the use of video files within <code><img></code> and <code><picture></code> elements. These also allow you to add in an <strong>Animated WebP</strong> for all other modern browsers.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">html</span></div><pre class=\"brush: html notranslate\"><code><picture>\n <source type=\"video/mp4\" src=\"giphy.mp4\" />\n <source type=\"image/webp\" src=\"giphy.webp\" />\n <img src=\"giphy.gif\" alt=\"A GIF animation\" />\n</picture>\n</code></pre></div>\n<h4 id=\"serving_the_optimal_size\">Serving the optimal size</h4>\n<p>In image delivery the \"one size fits all\" approach will not yield the best results, meaning that for smaller screens you would want to serve images with smaller resolution and vice versa for larger screens. On top of that, you'd also want to serve higher resolution images to those devices that boast a high DPI screen (e.g. \"Retina\"). So apart from creating plenty of intermediate image variants you also need a way to serve the right file to the right browser. That's where you would need to upgrade your <code><picture></code> and <code><source></code> elements with <a href=\"/en-US/docs/Web/HTML/Element/source#media\">media</a> and/or <a href=\"/en-US/docs/Web/HTML/Element/source#sizes\">sizes</a> attributes. A detailed article on how to combine all of these attributes can be found <a href=\"https://www.smashingmagazine.com/2014/05/responsive-images-done-right-guide-picture-srcset/\" class=\"external\" target=\"_blank\">here</a>.</p>\n<p>Two interesting effects to keep in mind regarding high dpi screens is that:</p>\n<ul>\n<li>with a high DPI screen, humans will spot compression artifacts a lot later, meaning that for images meant for these screens you can crank up compression beyond usual.</li>\n<li><a href=\"https://observablehq.com/@eeeps/visual-acuity-and-device-pixel-ratio\" class=\"external\" target=\"_blank\">Only a very few people can spot an increase in resolution beyond 2× DPI</a>, which means you don't need to serve images resolving higher than 2×.</li>\n</ul>\n<h4 id=\"controlling_the_priority_and_ordering_of_downloading_images\">Controlling the priority (and ordering) of downloading images</h4>\n<p>Getting the most important images in front of visitors sooner than the less important can deliver improved perceived performance.</p>\n<p>The first thing to check is that your content images use <code><img></code> or <code><picture></code> elements and your background images are defined in CSS with <code>background-image</code> — images referenced in <code><img></code> elements are assigned a higher loading priority than background images.</p>\n<p>Secondly, with the adoption of Priority Hints, you can control the priority further by adding a <code>fetchPriority</code> attribute to your image tags. An example use case for priority hints on images are carousels where the first image is a higher priority than the subsequent images.</p>"}},{"type":"prose","value":{"id":"rendering_strategy_preventing_jank_when_loading_images","title":"Rendering strategy: preventing jank when loading images","isH3":true,"content":"<p>As images are loaded asynchronously and continue to load after the first paint, if their dimensions aren't defined before load, they can cause reflows to the page content. For example, when text gets pushed down the page by images loading. For this reason, it's important to set <code>width</code> and <code>height</code> attributes so that the browser can reserve space for them in the layout.</p>\n<p>When the <code>width</code> and <code>height</code> attributes of an image are included on an HTML <a href=\"/en-US/docs/Web/HTML/Element/img\"><code><img></code></a> element, the <a href=\"/en-US/docs/Web/CSS/CSS_box_sizing/Understanding_aspect-ratio#adjusting_aspect_ratios_of_replaced_elements\">aspect ratio of the image</a> can be calculated by the browser prior to the image being loaded. This <a href=\"/en-US/docs/Glossary/Aspect_ratio\">aspect ratio</a> is used to reserve the space needed to display the image, reducing or even preventing a layout shift when the image is downloaded and painted to the screen. Reducing layout shift is a major component of good user experience and web performance.</p>\n<p>Browsers begin rendering content as HTML is parsed, often before all assets, including images, are downloaded. Including dimensions enable browsers to reserve a correctly-sized placeholder box for each image to appear in when the images are loaded when first rendering the page.</p>\n<p><img src=\"/en-US/docs/Learn_web_development/Extensions/Performance/Multimedia/ar-guide.jpg\" alt=\"Two screenshots the first without an image but with space reserved, the second showing the image loaded into the reserved space.\" width=\"1200\" height=\"857\" loading=\"lazy\"></p>\n<p>Without the <code>width</code> and <code>height</code> attributes, no placeholder space is created, creating a noticeable <a href=\"/en-US/docs/Glossary/Jank\">jank</a>, or layout shift, in the page when the image loads after the page is rendered. Page reflow and repaints are performance and usability issues.</p>\n<p>The <a href=\"/en-US/docs/Glossary/CLS\">CLS</a> metric measures jank on page load, or how much visible content shifts in the viewport and by how much. The main culprits of bad CLS are replaced elements without declared dimensions that reflow when the asset loads, including images, ad, embeds, and iframes without an size or <a href=\"/en-US/docs/Web/CSS/aspect-ratio\"><code>aspect-ratio</code></a> and web fonts.</p>\n<p>In responsive designs, when a container is narrower than an image, the following CSS is generally used to keep images from breaking out of their containers:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">css</span></div><pre class=\"brush: css notranslate\"><code>img {\n max-width: 100%;\n height: auto;\n}\n</code></pre></div>\n<p>While useful for responsive layouts, this causes jank and poor CLS when width and height information are not included, as if no height information is present when the <code><img></code> element is parsed but before the image has loaded, this CSS effectively has set the height to 0. When the image loads after the page has been initially rendered to the screen, the page reflows and repaints creating a layout shift as it creates space for the newly determined height.</p>\n<p>Browsers have a mechanism for sizing images before the actual image is loaded. When an <code><img></code>, <code><video></code>, or <code><input type=\"button\"></code> element has <code>width</code> and <code>height</code> attributes set on it, its aspect ratio is calculated before load time, and is available to the browser, using the dimensions provided.</p>\n<p>The aspect ratio is then used to calculate the height, and therefore, the correct size is applied to the <code><img></code> element, meaning that the aforementioned jank will not occur or be minimal if the listed dimensions are not fully accurate when the image loads.</p>\n<p>The aspect ratio is used to reserve space only on the image load. Once the image has loaded, the intrinsic aspect ratio of the loaded image or the value of the <code>aspect-ratio</code> property is used rather than the aspect ratio from the attributes. This ensures that it displays at the correct aspect ratio even if the attribute dimensions are not accurate.</p>\n<p>While developer best practices from the last decade may have recommended omitting the <code>width</code> and <code>height</code> attributes of an image on an HTML <a href=\"/en-US/docs/Web/HTML/Element/img\"><code><img></code></a>, due to aspect ratio mapping, including these two attributes is considered a developer best practice.</p>\n<p>For any background images, it's important you set a <code>background-color</code> value so any content overlaid is still readable before the image has downloaded.</p>"}},{"type":"prose","value":{"id":"conclusion","title":"Conclusion","isH3":false,"content":"<p>In this section, we took a look at image optimization. You now have a general understanding of how to optimize half of the average website's average bandwidth total. This is just one of the types of media consuming users' bandwidth and slowing down page load. Let's take a look at video optimization, tackling the next 20% of bandwidth consumption.</p>\n<ul class=\"prev-next\"><li><a class=\"button secondary\" href=\"/en-US/docs/Learn_web_development/Extensions/Performance/Measuring_performance\"><span class=\"button-wrap\"> Previous </span></a></li><li><a class=\"button secondary\" href=\"/en-US/docs/Learn_web_development/Extensions/Performance\"><span class=\"button-wrap\"> Overview: Web performance</span></a></li><li><a class=\"button secondary\" href=\"/en-US/docs/Learn_web_development/Extensions/Performance/video\"><span class=\"button-wrap\"> Next </span></a></li></ul>"}}],"isActive":true,"isMarkdown":true,"isTranslated":false,"locale":"en-US","mdn_url":"/en-US/docs/Learn_web_development/Extensions/Performance/Multimedia","modified":"2025-02-03T15:11:02.000Z","native":"English (US)","noIndexing":false,"other_translations":[{"locale":"de","title":"Multimedia: Bilder","native":"Deutsch"},{"locale":"fr","title":"Multimédia : Images","native":"Français"},{"locale":"ja","title":"マルチメディア: 画像","native":"日本語"},{"locale":"ko","title":"멀티미디어: 이미지","native":"한국어"},{"locale":"zh-CN","title":"多媒体：图片","native":"中文 (简体)"},{"locale":"zh-TW","title":"多媒體：圖像跟影片","native":"正體中文 (繁體)"}],"pageTitle":"Multimedia: Images - Learn web development | MDN","parents":[{"uri":"/en-US/docs/Learn_web_development","title":"Learn"},{"uri":"/en-US/docs/Learn_web_development/Extensions","title":"Extension modules"},{"uri":"/en-US/docs/Learn_web_development/Extensions/Performance","title":"Web performance"},{"uri":"/en-US/docs/Learn_web_development/Extensions/Performance/Multimedia","title":"Multimedia: Images"}],"popularity":null,"short_title":"Multimedia: Images","sidebarHTML":"<ol><li class=\"section\"><a href=\"/en-US/docs/Learn_web_development/Getting_started\">Getting started modules</a></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Getting_started/Environment_setup\">Environment setup</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Environment_setup/Installing_software\">Installing basic software</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Environment_setup/Browsing_the_web\">Browsing the web</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Environment_setup/Code_editors\">Code editors</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Environment_setup/Dealing_with_files\">Dealing with files</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Environment_setup/Command_line\">Command line crash course</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Getting_started/Your_first_website\">Your first website</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Your_first_website/What_will_your_website_look_like\">What will your website look like?</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Your_first_website/Creating_the_content\">HTML: Creating the content</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Your_first_website/Styling_the_content\">CSS: Styling the content</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Your_first_website/Adding_interactivity\">JavaScript: Adding interactivity</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Your_first_website/Publishing_your_website\">Publishing your website</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Getting_started/Web_standards\">Web standards</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Web_standards/How_the_web_works\">How the web works</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Web_standards/The_web_standards_model\">The web standards model</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Web_standards/How_browsers_load_websites\">How browsers load websites</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Getting_started/Soft_skills\">Soft skills</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Soft_skills/Research_and_learning\">Research and learning</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Soft_skills/Collaboration_and_teamwork\">Collaboration and teamwork</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Soft_skills/Workflows_and_processes\">Workflows and processes</a></li><li><a href=\"/en-US/docs/Learn_web_development/Getting_started/Soft_skills/Job_interviews\">Succeeding in job interviews</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn_web_development/Core\">Core modules</a></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content\">Structuring content with HTML</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Basic_HTML_syntax\">Basic HTML syntax</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Webpage_metadata\">What's in the head? Webpage metadata</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Headings_and_paragraphs\">Headings and paragraphs in HTML</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Emphasis_and_importance\">Emphasis and importance</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Lists\">Lists</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Structuring_documents\">Structuring documents</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Advanced_text_features\">Advanced text features</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Creating_links\">Creating links</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Marking_up_a_letter\">Challenge: Marking up a letter</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Structuring_a_page_of_content\">Challenge: Structuring a page of content</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/HTML_images\">HTML images</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/HTML_video_and_audio\">HTML video and audio</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Mozilla_splash_page\">Challenge: Mozilla splash page</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/HTML_table_basics\">HTML table basics</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Table_accessibility\">HTML table accessibility</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Planet_data_table\">Challenge: Structuring a planet data table</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/HTML_forms\">Forms and buttons in HTML</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Structuring_content/Debugging_HTML\">Debugging HTML</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics\">CSS styling basics</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/What_is_CSS\">What is CSS?</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Getting_started\">Getting started with CSS</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Styling_a_bio_page\">Challenge: Styling a biography page</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Basic_selectors\">Basic CSS selectors</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Attribute_selectors\">Attribute selectors</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Pseudo_classes_and_elements\">Pseudo-classes and pseudo-elements</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Combinators\">Combinators</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Box_model\">The box model</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Handling_conflicts\">Handling conflicts</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Values_and_units\">CSS values and units</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Sizing\">Sizing items in CSS</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Backgrounds_and_borders\">Backgrounds and borders</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Overflow\">Overflowing content</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Images_media_forms\">Images, media, and form elements</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Tables\">Styling tables</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Debugging_CSS\">Debugging CSS</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Fundamental_CSS_comprehension\">Challenge: Fundamental CSS comprehension</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Fancy_letterheaded_paper\">Challenge: Creating fancy letterheaded paper</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Styling_basics/Cool-looking_box\">Challenge: A cool-looking box</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Core/Text_styling\">CSS text styling</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Core/Text_styling/Fundamentals\">Fundamental text and font styling</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Text_styling/Styling_lists\">Styling lists</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Text_styling/Styling_links\">Styling links</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Text_styling/Web_fonts\">Web fonts</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Text_styling/Typesetting_a_homepage\">Challenge: Typesetting a community school homepage</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout\">CSS layout</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout/Introduction\">Introduction to CSS layout</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout/Floats\">Floats</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout/Positioning\">Positioning</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout/Flexbox\">Flexbox</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout/Grids\">CSS grid layout</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout/Responsive_Design\">Responsive design</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout/Media_queries\">Media query fundamentals</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/CSS_layout/Fundamental_Layout_Comprehension\">Challenge: Fundamental layout comprehension</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Core/Scripting\">Dynamic scripting with JavaScript</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/What_is_JavaScript\">What is JavaScript?</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/A_first_splash\">A first splash into JavaScript</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/What_went_wrong\">What went wrong? Troubleshooting JavaScript</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Variables\">Storing the information you need — Variables</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Math\">Basic math in JavaScript — numbers and operators</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Strings\">Handling text — strings in JavaScript</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Useful_string_methods\">Useful string methods</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Arrays\">Arrays</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Silly_story_generator\">Challenge: Silly story generator</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Conditionals\">Making decisions in your code — conditionals</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Loops\">Looping code</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Functions\">Functions — reusable blocks of code</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Build_your_own_function\">Build your own function</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Return_values\">Function return values</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Events\">Introduction to events</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Event_bubbling\">Event bubbling</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Image_gallery\">Challenge: Image gallery</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Object_basics\">JavaScript object basics</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/DOM_scripting\">DOM scripting introduction</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Network_requests\">Making network requests with JavaScript</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/JSON\">Working with JSON</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Scripting/Debugging_JavaScript\">Debugging JavaScript and handling errors</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries\">JavaScript frameworks and libraries</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/Introduction\">Introduction to client-side frameworks</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/Main_features\">Framework main features</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_getting_started\">Getting started with React</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_todo_list_beginning\">Beginning our React todo list</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_components\">Componentizing our React app</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_interactivity_events_state\">React interactivity: Events and state</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_interactivity_filtering_conditional_rendering\">React interactivity: Editing, filtering, conditional rendering</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_accessibility\">Accessibility in React</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Frameworks_libraries/React_resources\">React resources</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility\">Accessibility</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility/What_is_accessibility\">What is accessibility?</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility/Tooling\">Accessibility tooling and assistive technology</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility/HTML\">HTML: A good basis for accessibility</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility/CSS_and_JavaScript\">CSS and JavaScript accessibility best practices</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility/WAI-ARIA_basics\">WAI-ARIA basics</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility/Multimedia\">Accessible multimedia</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility/Mobile\">Mobile accessibility</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Accessibility/Accessibility_troubleshooting\">Challenge: Accessibility troubleshooting</a></li></ol></details></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Design_for_developers\">Design for developers</a></li><li><a href=\"/en-US/docs/Learn_web_development/Core/Version_control\">Version control</a></li><li class=\"section\"><a href=\"/en-US/docs/Learn_web_development/Extensions\">Extension modules</a></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects\">Advanced JavaScript objects</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Object_prototypes\">Object prototypes</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Object-oriented_programming\">Object-oriented programming</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Classes_in_JavaScript\">Classes in JavaScript</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Object_building_practice\">Object building practice</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Advanced_JavaScript_objects/Adding_bouncing_balls_features\">Challenge: Adding features to our bouncing balls demo</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_APIs\">Client-side web APIs</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_APIs/Introduction\">Introduction to web APIs</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_APIs/Video_and_audio_APIs\">Video and Audio APIs</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_APIs/Drawing_graphics\">Drawing graphics</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_APIs/Client-side_storage\">Client-side storage</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_APIs/Third_party_APIs\">Third-party APIs</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Async_JS\">Asynchronous JavaScript</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Async_JS/Introducing\">Introducing asynchronous JavaScript</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Async_JS/Promises\">How to use promises</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Async_JS/Implementing_a_promise-based_API\">How to implement a promise-based API</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Async_JS/Introducing_workers\">Introducing workers</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Async_JS/Sequencing_animations\">Challenge: Sequencing animations</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms\">Web forms</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/Your_first_form\">Your first form</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/How_to_structure_a_web_form\">How to structure a web form</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/Basic_native_form_controls\">Basic native form controls</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/HTML5_input_types\">The HTML5 input types</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/Other_form_controls\">Other form controls</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/Styling_web_forms\">Styling web forms</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/Advanced_form_styling\">Advanced form styling</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/UI_pseudo-classes\">UI pseudo-classes</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/Form_validation\">Client-side form validation</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Forms/Sending_and_retrieving_form_data\">Sending form data</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_tools\">Understanding client-side tools</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_tools/Overview\">Client-side tooling overview</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_tools/Package_management\">Package management basics</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_tools/Introducing_complete_toolchain\">Introducing a complete toolchain</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Client-side_tools/Deployment\">Deploying our app</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side\">Server-side websites</a></summary><ol><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/First_steps\">Server-side first steps</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/First_steps/Introduction\">Introduction to the server side</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/First_steps/Client-Server_overview\">Client-Server Overview</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/First_steps/Web_frameworks\">Server-side web frameworks</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/First_steps/Website_security\">Website security</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django\">Django web framework (Python)</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Introduction\">Django introduction</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/development_environment\">Setting up a Django development environment</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Tutorial_local_library_website\">Django Tutorial: The Local Library website</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/skeleton_website\">Django Tutorial Part 2: Creating a skeleton website</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Models\">Django Tutorial Part 3: Using models</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Admin_site\">Django Tutorial Part 4: Django admin site</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Home_page\">Django Tutorial Part 5: Creating our home page</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Generic_views\">Django Tutorial Part 6: Generic list and detail views</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Sessions\">Django Tutorial Part 7: Sessions framework</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Authentication\">Django Tutorial Part 8: User authentication and permissions</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Forms\">Django Tutorial Part 9: Working with forms</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Testing\">Django Tutorial Part 10: Testing a Django web application</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/Deployment\">Django Tutorial Part 11: Deploying Django to production</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/web_application_security\">Django web application security</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Django/django_assessment_blog\">Assessment: DIY Django mini blog</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs\">Express web framework (Node.js)</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/Introduction\">Express/Node introduction</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/development_environment\">Setting up a Node development environment</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/Tutorial_local_library_website\">Express Tutorial: The Local Library website</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/skeleton_website\">Express Tutorial Part 2: Creating a skeleton website</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/mongoose\">Express Tutorial Part 3: Using a Database (with Mongoose)</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/routes\">Express Tutorial Part 4: Routes and controllers</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/Displaying_data\">Express Tutorial Part 5: Displaying library data</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/forms\">Express Tutorial Part 6: Working with forms</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Server-side/Express_Nodejs/deployment\">Express Tutorial Part 7: Deploying to production</a></li></ol></details></li></ol></details></li><li class=\"toggle\"><details open=\"\"><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance\">Web performance</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/why_web_performance\">The \"why\" of web performance</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/What_is_web_performance\">What is web performance?</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/Perceived_performance\">Perceived performance</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/Measuring_performance\">Measuring performance</a></li><li><em><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/Multimedia\" aria-current=\"page\">Multimedia: Images</a></em></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/video\">Multimedia: video</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/JavaScript\">JavaScript performance optimization</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/HTML\">HTML performance optimization</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/CSS\">CSS performance optimization</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Performance/business_case_for_performance\">The business case for web performance</a></li></ol></details></li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Extensions/Testing\">Testing</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Testing/Introduction\">Introduction to cross-browser testing</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Testing/Testing_strategies\">Strategies for carrying out testing</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Testing/HTML_and_CSS\">Handling common HTML and CSS problems</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Testing/Feature_detection\">Implementing feature detection</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Testing/Automated_testing\">Introduction to automated testing</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Testing/Your_own_automation_environment\">Setting up your own test automation environment</a></li></ol></details></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Transform_animate\">Transform and animate CSS</a></li><li><a href=\"/en-US/docs/Learn_web_development/Extensions/Security_privacy\">Security and privacy</a></li><li class=\"section\">Further resources</li><li class=\"toggle\"><details><summary><a href=\"/en-US/docs/Learn_web_development/Howto\">How to solve common problems</a></summary><ol><li><a href=\"/en-US/docs/Learn_web_development/Howto/Solve_HTML_problems\">Solve common HTML problems</a></li><li><a href=\"/en-US/docs/Learn_web_development/Howto/Solve_CSS_problems\">Solve common CSS problems</a></li><li><a href=\"/en-US/docs/Learn_web_development/Howto/Solve_JavaScript_problems\">Solve common JavaScript problems</a></li><li><a href=\"/en-US/docs/Learn_web_development/Howto/Web_mechanics\">Web mechanics</a></li><li><a href=\"/en-US/docs/Learn_web_development/Howto/Tools_and_setup\">Tools and setup</a></li><li><a href=\"/en-US/docs/Learn_web_development/Howto/Design_and_accessibility\">Design and accessibility</a></li></ol></details></li><li><a href=\"/en-US/docs/Learn_web_development/About\">About</a></li><li><a href=\"/en-US/docs/Learn_web_development/Educators\">Resources for educators</a></li><li><a href=\"/en-US/docs/Learn_web_development/Changelog\">Changelog</a></li></ol>","source":{"folder":"en-us/learn_web_development/extensions/performance/multimedia","github_url":"https://github.com/mdn/content/blob/main/files/en-us/learn_web_development/extensions/performance/multimedia/index.md","last_commit_url":"https://github.com/mdn/content/commit/27bceead8e9b1fe9c92df0fa5e418f81bd5b9fdf","filename":"index.md"},"summary":"Media, namely images and video, account for over 70% of the bytes downloaded for the average website. In terms of download performance, eliminating media, and reducing file size is the low-hanging fruit. This article looks at optimizing images and video to improve web performance.","title":"Multimedia: Images","toc":[{"text":"Why optimize your multimedia?","id":"why_optimize_your_multimedia"},{"text":"Optimizing image delivery","id":"optimizing_image_delivery"},{"text":"Conclusion","id":"conclusion"}],"pageType":"learn-module-chapter"}}</script></body></html>