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Metadata in HTML</a></li><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/HTML_text_fundamentals">HTML text fundamentals</a></li><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Creating_hyperlinks">Creating hyperlinks</a></li><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Advanced_text_formatting">Advanced text formatting</a></li><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Document_and_website_structure">Document and website structure</a></li><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Debugging_HTML">Debugging HTML</a></li><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Marking_up_a_letter">Marking up a letter</a></li><li><a href="/en-US/docs/Learn/HTML/Introduction_to_HTML/Structuring_a_page_of_content">Structuring a page of content</a></li></ol></details></li><li><details><summary>Multimedia and embedding</summary><ol><li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding">Multimedia and embedding</a></li><li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Images_in_HTML">Images in HTML</a></li><li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Video_and_audio_content">Video and audio content</a></li><li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Other_embedding_technologies">From object to iframe — other embedding technologies</a></li><li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Adding_vector_graphics_to_the_Web">Adding vector graphics to the web</a></li><li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Responsive_images">Responsive images</a></li><li><a href="/en-US/docs/Learn/HTML/Multimedia_and_embedding/Mozilla_splash_page">Mozilla splash page</a></li></ol></details></li><li><details><summary>HTML tables</summary><ol><li><a href="/en-US/docs/Learn/HTML/Tables">HTML tables</a></li><li><a href="/en-US/docs/Learn/HTML/Tables/Basics">HTML table basics</a></li><li><a href="/en-US/docs/Learn/HTML/Tables/Advanced">HTML table advanced features and accessibility</a></li><li><a href="/en-US/docs/Learn/HTML/Tables/Structuring_planet_data">Structuring planet data</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/CSS">CSS — Styling the web</a></li><li><details><summary>CSS first steps</summary><ol><li><a href="/en-US/docs/Learn/CSS/First_steps">CSS first steps</a></li><li><a href="/en-US/docs/Learn/CSS/First_steps/What_is_CSS">What is CSS?</a></li><li><a href="/en-US/docs/Learn/CSS/First_steps/Getting_started">Getting started with CSS</a></li><li><a href="/en-US/docs/Learn/CSS/First_steps/How_CSS_is_structured">How CSS is structured</a></li><li><a href="/en-US/docs/Learn/CSS/First_steps/How_CSS_works">How CSS works</a></li><li><a href="/en-US/docs/Learn/CSS/First_steps/Styling_a_biography_page">Styling a biography page</a></li></ol></details></li><li><details><summary>CSS building blocks</summary><ol><li><a href="/en-US/docs/Learn/CSS/Building_blocks">CSS building blocks</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Selectors">CSS selectors</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Selectors/Type_Class_and_ID_Selectors">Type, class, and ID selectors</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Selectors/Attribute_selectors">Attribute selectors</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Selectors/Pseudo-classes_and_pseudo-elements">Pseudo-classes and pseudo-elements</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Selectors/Combinators">Combinators</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Cascade_and_inheritance">Cascade, specificity, and inheritance</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Cascade_layers">Cascade layers</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/The_box_model">The box model</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Backgrounds_and_borders">Backgrounds and borders</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Handling_different_text_directions">Handling different text directions</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Overflowing_content">Overflowing content</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Values_and_units">CSS values and units</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Sizing_items_in_CSS">Sizing items in CSS</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Images_media_form_elements">Images, media, and form elements</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Styling_tables">Styling tables</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Advanced_styling_effects">Advanced styling effects</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Debugging_CSS">Debugging CSS</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Organizing">Organizing your CSS</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Fundamental_CSS_comprehension">Fundamental CSS comprehension</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/Creating_fancy_letterheaded_paper">Creating fancy letterheaded paper</a></li><li><a href="/en-US/docs/Learn/CSS/Building_blocks/A_cool_looking_box">A cool-looking box</a></li></ol></details></li><li><details><summary>Styling text</summary><ol><li><a href="/en-US/docs/Learn/CSS/Styling_text">CSS styling text</a></li><li><a href="/en-US/docs/Learn/CSS/Styling_text/Fundamentals">Fundamental text and font styling</a></li><li><a href="/en-US/docs/Learn/CSS/Styling_text/Styling_lists">Styling lists</a></li><li><a href="/en-US/docs/Learn/CSS/Styling_text/Styling_links">Styling links</a></li><li><a href="/en-US/docs/Learn/CSS/Styling_text/Web_fonts">Web fonts</a></li><li><a href="/en-US/docs/Learn/CSS/Styling_text/Typesetting_a_homepage">Typesetting a community school homepage</a></li></ol></details></li><li><details><summary>CSS layout</summary><ol><li><a href="/en-US/docs/Learn/CSS/CSS_layout">CSS layout</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Introduction">Introduction to CSS layout</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Normal_Flow">Normal Flow</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Flexbox">Flexbox</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Grids">Grids</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Floats">Floats</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Positioning">Positioning</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Multiple-column_Layout">Multiple-column layout</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Responsive_Design">Responsive design</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Media_queries">Beginner's guide to media queries</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Legacy_Layout_Methods">Legacy layout methods</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Supporting_Older_Browsers">Supporting older browsers</a></li><li><a href="/en-US/docs/Learn/CSS/CSS_layout/Fundamental_Layout_Comprehension">Fundamental layout comprehension</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/JavaScript">JavaScript — Dynamic client-side scripting</a></li><li><details><summary>JavaScript first steps</summary><ol><li><a href="/en-US/docs/Learn/JavaScript/First_steps">JavaScript first steps</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/What_is_JavaScript">What is JavaScript?</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/A_first_splash">A first splash into JavaScript</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/What_went_wrong">What went wrong? Troubleshooting JavaScript</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/Variables">Storing the information you need — Variables</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/Math">Basic math in JavaScript — numbers and operators</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/Strings">Handling text — strings in JavaScript</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/Useful_string_methods">Useful string methods</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/Arrays">Arrays</a></li><li><a href="/en-US/docs/Learn/JavaScript/First_steps/Silly_story_generator">Silly story generator</a></li></ol></details></li><li><details><summary>JavaScript building blocks</summary><ol><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks">JavaScript building blocks</a></li><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/conditionals">Making decisions in your code — conditionals</a></li><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/Looping_code">Looping code</a></li><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/Functions">Functions — reusable blocks of code</a></li><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/Build_your_own_function">Build your own function</a></li><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/Return_values">Function return values</a></li><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/Events">Introduction to events</a></li><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/Event_bubbling">Event bubbling</a></li><li><a href="/en-US/docs/Learn/JavaScript/Building_blocks/Image_gallery">Image gallery</a></li></ol></details></li><li><details><summary>Introducing JavaScript objects</summary><ol><li><a href="/en-US/docs/Learn/JavaScript/Objects">Introducing JavaScript objects</a></li><li><a href="/en-US/docs/Learn/JavaScript/Objects/Basics">JavaScript object basics</a></li><li><a href="/en-US/docs/Learn/JavaScript/Objects/Object_prototypes">Object prototypes</a></li><li><a href="/en-US/docs/Learn/JavaScript/Objects/Object-oriented_programming">Object-oriented programming</a></li><li><a href="/en-US/docs/Learn/JavaScript/Objects/Classes_in_JavaScript">Classes in JavaScript</a></li><li><a href="/en-US/docs/Learn/JavaScript/Objects/JSON">Working with JSON</a></li><li><a href="/en-US/docs/Learn/JavaScript/Objects/Object_building_practice">Object building practice</a></li><li><a href="/en-US/docs/Learn/JavaScript/Objects/Adding_bouncing_balls_features">Adding features to our bouncing balls demo</a></li></ol></details></li><li><details><summary>Asynchronous JavaScript</summary><ol><li><a href="/en-US/docs/Learn/JavaScript/Asynchronous">Asynchronous JavaScript</a></li><li><a href="/en-US/docs/Learn/JavaScript/Asynchronous/Introducing">Introducing asynchronous JavaScript</a></li><li><a href="/en-US/docs/Learn/JavaScript/Asynchronous/Promises">How to use promises</a></li><li><a href="/en-US/docs/Learn/JavaScript/Asynchronous/Implementing_a_promise-based_API">How to implement a promise-based API</a></li><li><a href="/en-US/docs/Learn/JavaScript/Asynchronous/Introducing_workers">Introducing workers</a></li><li><a href="/en-US/docs/Learn/JavaScript/Asynchronous/Sequencing_animations">Sequencing animations</a></li></ol></details></li><li><details><summary>Client-side web APIs</summary><ol><li><a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs">Client-side web APIs</a></li><li><a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Introduction">Introduction to web APIs</a></li><li><a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Manipulating_documents">Manipulating documents</a></li><li><a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Fetching_data">Fetching data from the server</a></li><li><a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Third_party_APIs">Third-party APIs</a></li><li><a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Drawing_graphics">Drawing graphics</a></li><li><a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Video_and_audio_APIs">Video and Audio APIs</a></li><li><a href="/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Client-side_storage">Client-side storage</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/Forms">Web forms — Working with user data</a></li><li><details><summary>Web form building blocks</summary><ol><li><a href="/en-US/docs/Learn/Forms">Web form building blocks</a></li><li><a href="/en-US/docs/Learn/Forms/Your_first_form">Your first form</a></li><li><a href="/en-US/docs/Learn/Forms/How_to_structure_a_web_form">How to structure a web form</a></li><li><a href="/en-US/docs/Learn/Forms/Basic_native_form_controls">Basic native form controls</a></li><li><a href="/en-US/docs/Learn/Forms/HTML5_input_types">The HTML5 input types</a></li><li><a href="/en-US/docs/Learn/Forms/Other_form_controls">Other form controls</a></li><li><a href="/en-US/docs/Learn/Forms/Styling_web_forms">Styling web forms</a></li><li><a href="/en-US/docs/Learn/Forms/Advanced_form_styling">Advanced form styling</a></li><li><a href="/en-US/docs/Learn/Forms/UI_pseudo-classes">UI pseudo-classes</a></li><li><a href="/en-US/docs/Learn/Forms/Form_validation">Client-side form validation</a></li><li><a href="/en-US/docs/Learn/Forms/Sending_and_retrieving_form_data">Sending form data</a></li></ol></details></li><li><details><summary>Advanced web form techniques</summary><ol><li><a href="/en-US/docs/Learn/Forms/How_to_build_custom_form_controls">How to build custom form controls</a></li><li><a href="/en-US/docs/Learn/Forms/Sending_forms_through_JavaScript">Sending forms through JavaScript</a></li><li><a href="/en-US/docs/Learn/Forms/Property_compatibility_table_for_form_controls">CSS property compatibility table for form controls</a></li><li><a href="/en-US/docs/Learn/Forms/HTML_forms_in_legacy_browsers">HTML forms in legacy browsers</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/Accessibility">Accessibility — Make the web usable by everyone</a></li><li><details><summary>Accessibility guides</summary><ol><li><a href="/en-US/docs/Learn/Accessibility">Accessibility</a></li><li><a href="/en-US/docs/Learn/Accessibility/What_is_accessibility">What is accessibility?</a></li><li><a href="/en-US/docs/Learn/Accessibility/HTML">HTML: A good basis for accessibility</a></li><li><a href="/en-US/docs/Learn/Accessibility/CSS_and_JavaScript">CSS and JavaScript accessibility best practices</a></li><li><a href="/en-US/docs/Learn/Accessibility/WAI-ARIA_basics">WAI-ARIA basics</a></li><li><a href="/en-US/docs/Learn/Accessibility/Multimedia">Accessible multimedia</a></li><li><a href="/en-US/docs/Learn/Accessibility/Mobile">Mobile accessibility</a></li><li><a href="/en-US/docs/Learn/Accessibility/Accessibility_troubleshooting">Assessment: Accessibility troubleshooting</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/Performance">Performance — Making websites fast and responsive</a></li><li><details open=""><summary>Performance guides</summary><ol><li><a href="/en-US/docs/Learn/Performance">Web performance</a></li><li><a href="/en-US/docs/Learn/Performance/why_web_performance">The "why" of web performance</a></li><li><a href="/en-US/docs/Learn/Performance/What_is_web_performance">What is web performance?</a></li><li><a href="/en-US/docs/Learn/Performance/Perceived_performance">Perceived performance</a></li><li><a href="/en-US/docs/Learn/Performance/Measuring_performance">Measuring performance</a></li><li><a href="/en-US/docs/Learn/Performance/Multimedia">Multimedia: Images</a></li><li><a href="/en-US/docs/Learn/Performance/video">Multimedia: video</a></li><li><a href="/en-US/docs/Learn/Performance/JavaScript">JavaScript performance optimization</a></li><li><a href="/en-US/docs/Learn/Performance/HTML">HTML performance optimization</a></li><li><em><a href="/en-US/docs/Learn/Performance/CSS" aria-current="page">CSS performance optimization</a></em></li><li><a href="/en-US/docs/Learn/Performance/business_case_for_performance">The business case for web performance</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/MathML">MathML — Writing mathematics with MathML</a></li><li><details><summary>MathML first steps</summary><ol><li><a href="/en-US/docs/Learn/MathML/First_steps">MathML first steps</a></li><li><a href="/en-US/docs/Learn/MathML/First_steps/Getting_started">Getting started with MathML</a></li><li><a href="/en-US/docs/Learn/MathML/First_steps/Text_containers">MathML Text Containers</a></li><li><a href="/en-US/docs/Learn/MathML/First_steps/Fractions_and_roots">MathML fractions and roots</a></li><li><a href="/en-US/docs/Learn/MathML/First_steps/Scripts">MathML scripted elements</a></li><li><a href="/en-US/docs/Learn/MathML/First_steps/Tables">MathML tables</a></li><li><a href="/en-US/docs/Learn/MathML/First_steps/Three_famous_mathematical_formulas">Three famous mathematical formulas</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/../Games">Games — Developing games for the web</a></li><li><details><summary>Guides and tutorials</summary><ol><li><a href="/en-US/docs/Games/Introduction">Introduction to game development for the Web</a></li><li><a href="/en-US/docs/Games/Techniques">Techniques for game development</a></li><li><a href="/en-US/docs/Games/Tutorials">Tutorials</a></li><li><a href="/en-US/docs/Games/Publishing_games">Publishing games</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/Tools_and_testing">Tools and testing</a></li><li><details><summary>Client-side web development tools</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools">Understanding client-side web development tools</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Overview">Client-side tooling overview</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Command_line">Command line crash course</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Package_management">Package management basics</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Introducing_complete_toolchain">Introducing a complete toolchain</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Deployment">Deploying our app</a></li></ol></details></li><li><details><summary>Introduction to client-side frameworks</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Introduction">Introduction to client-side frameworks</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Main_features">Framework main features</a></li></ol></details></li><li><details><summary>React</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_getting_started">Getting started with React</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_todo_list_beginning">Beginning our React todo list</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_components">Componentizing our React app</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_interactivity_events_state">React interactivity: Events and state</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_interactivity_filtering_conditional_rendering">React interactivity: Editing, filtering, conditional rendering</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_accessibility">Accessibility in React</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_resources">React resources</a></li></ol></details></li><li><details><summary>Ember</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_getting_started">Getting started with Ember</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_structure_componentization">Ember app structure and componentization</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_interactivity_events_state">Ember interactivity: Events, classes and state</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_conditional_footer">Ember Interactivity: Footer functionality, conditional rendering</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_routing">Routing in Ember</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_resources">Ember resources and troubleshooting</a></li></ol></details></li><li><details><summary>Vue</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_getting_started">Getting started with Vue</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_first_component">Creating our first Vue component</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_rendering_lists">Rendering a list of Vue components</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_methods_events_models">Adding a new todo form: Vue events, methods, and models</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_styling">Styling Vue components with CSS</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_computed_properties">Using Vue computed properties</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_conditional_rendering">Vue conditional rendering: editing existing todos</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_refs_focus_management">Vue refs and lifecycle methods for focus management</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_resources">Vue resources</a></li></ol></details></li><li><details><summary>Svelte</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_getting_started">Getting started with Svelte</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_Todo_list_beginning">Starting our Svelte to-do list app</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_variables_props">Dynamic behavior in Svelte: working with variables and props</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_components">Componentizing our Svelte app</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_reactivity_lifecycle_accessibility">Advanced Svelte: Reactivity, lifecycle, accessibility</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_stores">Working with Svelte stores</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_TypeScript">TypeScript support in Svelte</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_deployment_next">Deployment and next steps</a></li></ol></details></li><li><details><summary>Angular</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_getting_started">Getting started with Angular</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_todo_list_beginning">Beginning our Angular todo list app</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_styling">Styling our Angular app</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_item_component">Creating an item component</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_filtering">Filtering our to-do items</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_building">Building Angular applications and further resources</a></li></ol></details></li><li><details><summary>Git and GitHub</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/GitHub">Git and GitHub</a></li></ol></details></li><li><details><summary>Cross browser testing</summary><ol><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing">Cross browser testing</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Introduction">Introduction to cross-browser testing</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Testing_strategies">Strategies for carrying out testing</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/HTML_and_CSS">Handling common HTML and CSS problems</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/JavaScript">Handling common JavaScript problems</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Accessibility">Handling common accessibility problems</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Feature_detection">Implementing feature detection</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Automated_testing">Introduction to automated testing</a></li><li><a href="/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Your_own_automation_environment">Setting up your own test automation environment</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/Server-side">Server-side website programming</a></li><li><details><summary>First steps</summary><ol><li><a href="/en-US/docs/Learn/Server-side/First_steps">Server-side website programming first steps</a></li><li><a href="/en-US/docs/Learn/Server-side/First_steps/Introduction">Introduction to the server side</a></li><li><a href="/en-US/docs/Learn/Server-side/First_steps/Client-Server_overview">Client-Server Overview</a></li><li><a href="/en-US/docs/Learn/Server-side/First_steps/Web_frameworks">Server-side web frameworks</a></li><li><a href="/en-US/docs/Learn/Server-side/First_steps/Website_security">Website security</a></li></ol></details></li><li><details><summary>Django web framework (Python)</summary><ol><li><a href="/en-US/docs/Learn/Server-side/Django">Django Web Framework (Python)</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Introduction">Django introduction</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/development_environment">Setting up a Django development environment</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Tutorial_local_library_website">Django Tutorial: The Local Library website</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/skeleton_website">Django Tutorial Part 2: Creating a skeleton website</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Models">Django Tutorial Part 3: Using models</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Admin_site">Django Tutorial Part 4: Django admin site</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Home_page">Django Tutorial Part 5: Creating our home page</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Generic_views">Django Tutorial Part 6: Generic list and detail views</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Sessions">Django Tutorial Part 7: Sessions framework</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Authentication">Django Tutorial Part 8: User authentication and permissions</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Forms">Django Tutorial Part 9: Working with forms</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Testing">Django Tutorial Part 10: Testing a Django web application</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/Deployment">Django Tutorial Part 11: Deploying Django to production</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/web_application_security">Django web application security</a></li><li><a href="/en-US/docs/Learn/Server-side/Django/django_assessment_blog">Assessment: DIY Django mini blog</a></li></ol></details></li><li><details><summary>Express Web Framework (Node.js/JavaScript)</summary><ol><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs">Express web framework (Node.js/JavaScript)</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/Introduction">Express/Node introduction</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/development_environment">Setting up a Node development environment</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/Tutorial_local_library_website">Express Tutorial: The Local Library website</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/skeleton_website">Express Tutorial Part 2: Creating a skeleton website</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/mongoose">Express Tutorial Part 3: Using a Database (with Mongoose)</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/routes">Express Tutorial Part 4: Routes and controllers</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/Displaying_data">Express Tutorial Part 5: Displaying library data</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/forms">Express Tutorial Part 6: Working with forms</a></li><li><a href="/en-US/docs/Learn/Server-side/Express_Nodejs/deployment">Express Tutorial Part 7: Deploying to production</a></li></ol></details></li><li class="section"><a href="/en-US/docs/Learn/Common_questions">Further resources</a></li><li><details><summary>Common questions</summary><ol><li><a href="/en-US/docs/Learn/Common_questions">Common questions</a></li><li><a href="/en-US/docs/Learn/HTML/Howto">Use HTML to solve common problems</a></li><li><a href="/en-US/docs/Learn/CSS/Howto">Use CSS to solve common problems</a></li><li><a href="/en-US/docs/Learn/JavaScript/Howto">Solve common problems in your JavaScript code</a></li><li><a href="/en-US/docs/Learn/Common_questions/Web_mechanics">Web mechanics</a></li><li><a href="/en-US/docs/Learn/Common_questions/Tools_and_setup">Tools and setup</a></li><li><a href="/en-US/docs/Learn/Common_questions/Design_and_accessibility">Design and accessibility</a></li></ol></details></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="#to_optimize_or_not_to_optimize">To optimize or not to optimize</a></li><li class="document-toc-item "><a class="document-toc-link" href="#optimizing_rendering">Optimizing rendering</a></li><li class="document-toc-item "><a class="document-toc-link" href="#handling_animations">Handling animations</a></li><li class="document-toc-item "><a class="document-toc-link" href="#optimizing_element_changes_with_will-change">Optimizing element changes with <code>will-change</code></a></li><li class="document-toc-item "><a class="document-toc-link" href="#optimizing_for_render_blocking">Optimizing for render blocking</a></li><li class="document-toc-item "><a class="document-toc-link" href="#improving_font_performance">Improving font performance</a></li><li class="document-toc-item "><a class="document-toc-link" href="#optimizing_styling_recalculation_with_css_containment">Optimizing styling recalculation with CSS containment</a></li><li class="document-toc-item "><a class="document-toc-link" href="#see_also">See also</a></li></ul></section></div></nav></aside><section class="place side"></section></div></div><main id="content" class="main-content "><article class="main-page-content" lang="en-US"><header><h1>CSS performance optimization</h1></header><div class="section-content"><ul class="prev-next"> <li><a class="button secondary" href="/en-US/docs/Learn/Performance/HTML"><span class="button-wrap"> Previous </span></a></li> <li><a class="button secondary" href="/en-US/docs/Learn/Performance"><span class="button-wrap"> Overview: Web performance</span></a></li> <li><a class="button secondary" href="/en-US/docs/Learn/Performance/business_case_for_performance"><span class="button-wrap"> Next </span></a></li> </ul> <p>When developing a website, you need to consider how the browser is handling the CSS on your site. To mitigate any performance issues that CSS might be causing, you should optimize it. For example, you should optimize the CSS to mitigate <a href="/en-US/docs/Glossary/Render_blocking">render-blocking</a> and minimize the number of required reflows. This article walks you through key CSS performance optimization techniques.</p> <figure class="table-container"><table> <tbody> <tr> <th scope="row">Prerequisites:</th> <td> <a href="/en-US/docs/Learn/Getting_started_with_the_web/Installing_basic_software">Basic software installed</a>, and basic knowledge of <a href="/en-US/docs/Learn/Getting_started_with_the_web">client-side web technologies</a>. </td> </tr> <tr> <th scope="row">Objective:</th> <td> To learn about the impact of CSS on website performance and how to optimize your CSS to improve performance. </td> </tr> </tbody> </table></figure></div><section aria-labelledby="to_optimize_or_not_to_optimize"><h2 id="to_optimize_or_not_to_optimize"><a href="#to_optimize_or_not_to_optimize">To optimize or not to optimize</a></h2><div class="section-content"><p>The first question you should answer before starting to optimize your CSS is "what do I need to optimize?". Some of the tips and techniques discussed below are good practices that will benefit just about any web project, whereas some are only needed in certain situations. Trying to apply all these techniques everywhere is probably unnecessary, and may be a waste of your time. You should figure out what performance optimizations are actually needed in each project.</p> <p>To do this, you need to <a href="/en-US/docs/Learn/Performance/Measuring_performance">measure the performance</a> of your site. As the previous link shows, there are several different ways to measure performance, some involving sophisticated <a href="/en-US/docs/Web/API/Performance_API">performance APIs</a>. The best way to get started however, is to learn how to use tools such as built-in browser <a href="/en-US/docs/Learn/Performance/Measuring_performance#network_monitor_tools">network</a> and <a href="/en-US/docs/Learn/Performance/Measuring_performance#performance_monitor_tools">performance</a> tools, to see what parts of the page load are taking a long time and need optimizing.</p></div></section><section aria-labelledby="optimizing_rendering"><h2 id="optimizing_rendering"><a href="#optimizing_rendering">Optimizing rendering</a></h2><div class="section-content"><p>Browsers follow a specific rendering path — paint only occurs after layout, which occurs after the render tree is created, which in turn requires both the DOM and the CSSOM trees.</p> <p>Showing users an unstyled page and then repainting it after the CSS styles have been parsed would be a bad user experience. For this reason, CSS is render blocking until the browser determines that the CSS is required. The browser can paint the page after it has downloaded the CSS and built the <a href="/en-US/docs/Glossary/CSSOM">CSS object model (CSSOM)</a>.</p> <p>To optimize the CSSOM construction and improve page performance, you can do one or more of the following based on the current state of your CSS:</p> <ul> <li> <p><strong>Remove unnecessary styles</strong>: This may sound obvious, but it is surprising how many developers forget to clean up unused CSS rules that were added to their stylesheets during development and ended up not being used. All styles get parsed, whether they are being used during layout and painting or not, so it can speed up page rendering to get rid of unused ones. As <a href="https://css-tricks.com/how-do-you-remove-unused-css-from-a-site/" class="external" target="_blank">How Do You Remove Unused CSS From a Site?</a> (csstricks.com, 2019) summarizes, this is a difficult problem to solve for a large codebase, and there isn't a magic bullet to reliably find and remove unused CSS. You need to do the hard work of keeping your CSS modular and being careful and deliberate about what is added and removed.</p> </li> <li> <p><strong>Split CSS into separate modules</strong>: Keeping CSS modular means that CSS not required at page load can be loaded later on, reducing initial CSS render-blocking and loading times. The simplest way to do this is by splitting up your CSS into separate files and loading only what is needed:</p> <div class="code-example"><div class="example-header"><span class="language-name">html</span></div><pre class="brush: html notranslate"><code> <link rel="stylesheet" href="styles.css" />  <link rel="stylesheet" href="print.css" media="print" />  <link rel="stylesheet" href="mobile.css" media="screen and (max-width: 480px)" /> </code></pre></div> <p>The above example provides three sets of styles — default styles that will always load, styles that will only be loaded when the document is being printed, and styles that will be loaded only by devices with narrow screens. By default, the browser assumes that each specified style sheet is render-blocking. You can tell the browser when a style sheet should be applied by adding a <code>media</code> attribute containing a <a href="/en-US/docs/Web/CSS/CSS_media_queries/Using_media_queries">media query</a>. When the browser sees a style sheet that it only needs to apply in a specific scenario, it still downloads the stylesheet, but doesn't render-block. By separating the CSS into multiple files, the main render-blocking file, in this case <code>styles.css</code>, is much smaller, reducing the time for which rendering is blocked.</p> </li> <li> <p><strong>Minify and compress your CSS</strong>: Minifying involves removing all the whitespace in the file that is only there for human readability, once the code is put into production. You can reduce loading times considerably by minifying your CSS. Minification is generally done as part of a build process (for example, most JavaScript frameworks will minify code when you build a project ready for deployment). In addition to minification, make sure that the server that your site is hosted on uses compression such as gzip on files before serving them.</p> </li> <li> <p><strong>Simplify selectors</strong>: People often write selectors that are more complex than needed for applying the required styles. This not only increases file sizes, but also the parsing time for those selectors. For example:</p> <div class="code-example"><div class="example-header"><span class="language-name">css</span></div><pre class="brush: css notranslate"><code>/* Very specific selector */ body div#main-content article.post h2.headline { font-size: 24px; } /* You probably only need this */ .headline { font-size: 24px; } </code></pre></div> <p>Making your selectors less complex and specific is also good for maintenance. It is easy to understand what simple selectors are doing, and it is easy to override styles when needed later on if the selectors are less <a href="/en-US/docs/Learn/CSS/Building_blocks/Cascade_and_inheritance#specificity_2">specific</a>.</p> </li> <li> <p><strong>Don't apply styles to more elements than needed</strong>: A common mistake is to apply styles to all elements using the <a href="/en-US/docs/Web/CSS/Universal_selectors">universal selector</a>, or at least, to more elements than needed. This kind of styling can impact performance negatively, especially on larger sites.</p> <div class="code-example"><div class="example-header"><span class="language-name">css</span></div><pre class="brush: css notranslate"><code>/* Selects every element inside the <body> */ body * { font-size: 14px; display: flex; } </code></pre></div> <p>Remember that many properties (such as <a href="/en-US/docs/Web/CSS/font-size"><code>font-size</code></a>) inherit their values from their parents, so you don't need to apply them everywhere. And powerful tools such as <a href="/en-US/docs/Learn/CSS/CSS_layout/Flexbox">Flexbox</a> need to be used sparingly. Using them everywhere can cause all kinds of unexpected behavior.</p> </li> <li> <p><strong>Cut down on image HTTP requests with CSS sprites</strong>: <a href="https://css-tricks.com/css-sprites/" class="external" target="_blank">CSS sprites</a> is a technique that places several small images (such as icons) that you want to use on your site into a single image file, and then uses different <a href="/en-US/docs/Web/CSS/background-position"><code>background-position</code></a> values to display the chunk of image that you want to show in each different place. This can dramatically cut down on the number of HTTP requests needed to fetch the images.</p> </li> <li> <p><strong>Preload important assets</strong>: You can use <a href="/en-US/docs/Web/HTML/Attributes/rel/preload"><code>rel="preload"</code></a> to turn <a href="/en-US/docs/Web/HTML/Element/link"><code><link></code></a> elements into preloaders for critical assets. This includes CSS files, fonts, and images:</p> <div class="code-example"><div class="example-header"><span class="language-name">html</span></div><pre class="brush: html notranslate"><code><link rel="preload" href="style.css" as="style" /> <link rel="preload" href="ComicSans.woff2" as="font" type="font/woff2" crossorigin /> <link rel="preload" href="bg-image-wide.png" as="image" media="(min-width: 601px)" /> </code></pre></div> <p>With <code>preload</code>, the browser will fetch the referenced resources as soon as possible and make them available in the browser cache so that they will be ready for use sooner when they are referenced in subsequent code. It is useful to preload high-priority resources that the user will encounter early on in a page so that the experience is as smooth as possible. Note how you can also use <code>media</code> attributes to create responsive preloaders.</p> <p>See also <a href="https://web.dev/articles/preload-critical-assets" class="external" target="_blank">Preload critical assets to improve loading speed</a> on web.dev (2020)</p> </li> </ul></div></section><section aria-labelledby="handling_animations"><h2 id="handling_animations"><a href="#handling_animations">Handling animations</a></h2><div class="section-content"><p>Animations can improve perceived performance, making interfaces feel snappier and making users feel like progress is being made when they are waiting for a page to load (loading spinners, for example). However, larger animations and a higher number of animations will naturally require more processing power to handle, which can degrade performance.</p> <p>The simplest advice is to cut down on all unnecessary animations. You could also provide users with a control/site preference to turn off animations if they are using a low-powered device or a mobile device with limited battery power. You could also use JavaScript to control whether or not animation is applied to the page in the first place. There is also a media query called <a href="/en-US/docs/Web/CSS/@media/prefers-reduced-motion"><code>prefers-reduced-motion</code></a> that can be used to selectively serve animation styles or not based on a user's OS-level preferences for animation.</p> <p>For essential DOM animations, you are advised to use <a href="/en-US/docs/Web/CSS/CSS_animations/Using_CSS_animations">CSS animations</a> where possible, rather than JavaScript animations (the <a href="/en-US/docs/Web/API/Web_Animations_API">Web Animations API</a> provides a way to directly hook into CSS animations using JavaScript).</p></div></section><section aria-labelledby="choosing_properties_to_animate"><h3 id="choosing_properties_to_animate"><a href="#choosing_properties_to_animate">Choosing properties to animate</a></h3><div class="section-content"><p>Next, animation performance relies heavily on what properties you are animating. Certain properties, when animated, trigger a <a href="/en-US/docs/Glossary/Reflow">reflow</a> (and therefore also a <a href="/en-US/docs/Glossary/Repaint">repaint</a>) and should be avoided. These include properties that:</p> <ul> <li>Alter an element's dimensions, such as <a href="/en-US/docs/Web/CSS/width"><code>width</code></a>, <a href="/en-US/docs/Web/CSS/height"><code>height</code></a>, <a href="/en-US/docs/Web/CSS/border"><code>border</code></a>, and <a href="/en-US/docs/Web/CSS/padding"><code>padding</code></a>.</li> <li>Reposition an element, such as <a href="/en-US/docs/Web/CSS/margin"><code>margin</code></a>, <a href="/en-US/docs/Web/CSS/top"><code>top</code></a>, <a href="/en-US/docs/Web/CSS/bottom"><code>bottom</code></a>, <a href="/en-US/docs/Web/CSS/left"><code>left</code></a>, and <a href="/en-US/docs/Web/CSS/right"><code>right</code></a>.</li> <li>Change an element's layout, such as <a href="/en-US/docs/Web/CSS/align-content"><code>align-content</code></a>, <a href="/en-US/docs/Web/CSS/align-items"><code>align-items</code></a>, and <a href="/en-US/docs/Web/CSS/flex"><code>flex</code></a>.</li> <li>Add visual effects that change the element geometry, such as <a href="/en-US/docs/Web/CSS/box-shadow"><code>box-shadow</code></a>.</li> </ul> <p>Modern browsers are smart enough to repaint only the changed area of the document, rather than the entire page. As a result, larger animations are more costly.</p> <p>If at all possible, it is better to animate properties that do not cause reflow/repaint. This includes:</p> <ul> <li><a href="/en-US/docs/Web/CSS/CSS_transforms">Transforms</a></li> <li><a href="/en-US/docs/Web/CSS/opacity"><code>opacity</code></a></li> <li><a href="/en-US/docs/Web/CSS/filter"><code>filter</code></a></li> </ul></div></section><section aria-labelledby="animating_on_the_gpu"><h3 id="animating_on_the_gpu"><a href="#animating_on_the_gpu">Animating on the GPU</a></h3><div class="section-content"><p>To further improve performance, you should consider moving animation work off the main thread and onto the device's GPU (also referred to as compositing). This is done by choosing specific types of animations that the browser will automatically send to the GPU to handle; these include:</p> <ul> <li>3D transform animations such as <a href="/en-US/docs/Web/CSS/transform"><code>transform: translateZ()</code></a> and <a href="/en-US/docs/Web/CSS/transform-function/rotate3d"><code>rotate3d()</code></a>.</li> <li>Elements with certain other properties animated such as <a href="/en-US/docs/Web/CSS/position"><code>position: fixed</code></a>.</li> <li>Elements with <a href="/en-US/docs/Web/CSS/will-change"><code>will-change</code></a> applied (see the section below).</li> <li>Certain elements that are rendered in their own layer, including <a href="/en-US/docs/Web/HTML/Element/video"><code><video></code></a>, <a href="/en-US/docs/Web/HTML/Element/canvas"><code><canvas></code></a>, and <a href="/en-US/docs/Web/HTML/Element/iframe"><code><iframe></code></a>.</li> </ul> <p>Animation on the GPU can result in improved performance, especially on mobile. However, moving animations to GPU is not always that simple. Read <a href="https://www.smashingmagazine.com/2016/12/gpu-animation-doing-it-right/" class="external" target="_blank">CSS GPU Animation: Doing It Right</a> (smashingmagazine.com, 2016) for a very useful and detailed analysis.</p></div></section><section aria-labelledby="optimizing_element_changes_with_will-change"><h2 id="optimizing_element_changes_with_will-change"><a href="#optimizing_element_changes_with_will-change">Optimizing element changes with <code>will-change</code></a></h2><div class="section-content"><p>Browsers may set up optimizations before an element is actually changed. These kinds of optimizations can increase the responsiveness of a page by doing potentially expensive work before it is required. The CSS <a href="/en-US/docs/Web/CSS/will-change"><code>will-change</code></a> property hints to browsers how an element is expected to change.</p> <div class="notecard note"> <p><strong>Note:</strong> <code>will-change</code> is intended to be used as a last resort to try to deal with existing performance problems. It should not be used to anticipate performance problems.</p> </div> <div class="code-example"><div class="example-header"><span class="language-name">css</span></div><pre class="brush: css notranslate"><code>.element { will-change: opacity, transform; } </code></pre></div></div></section><section aria-labelledby="optimizing_for_render_blocking"><h2 id="optimizing_for_render_blocking"><a href="#optimizing_for_render_blocking">Optimizing for render blocking</a></h2><div class="section-content"><p>CSS can scope styles to particular conditions with media queries. Media queries are important for a responsive web design and help us optimize a critical rendering path. The browser blocks rendering until it parses all of these styles but will not block rendering on styles it knows it will not use, such as the print stylesheets. By splitting the CSS into multiple files based on media queries, you can prevent render blocking during download of unused CSS. To create a non-blocking CSS link, move the not-immediately used styles, such as print styles, into separate file, add a <a href="/en-US/docs/Web/HTML/Element/link"><code><link></code></a> to the HTML mark up, and add a media query, in this case stating it's a print stylesheet.</p> <div class="code-example"><div class="example-header"><span class="language-name">html</span></div><pre class="brush: html notranslate"><code> <link rel="stylesheet" href="styles.css" />  <link rel="stylesheet" href="print.css" media="print" />  <link rel="stylesheet" href="mobile.css" media="screen and (max-width: 480px)" /> </code></pre></div> <p>By default, the browser assumes that each specified style sheet is render blocking. Tell the browser when the style sheet should be applied by adding a <code>media</code> attribute with the <a href="/en-US/docs/Web/CSS/CSS_media_queries/Using_media_queries">media query</a>. When the browser sees a style sheet it knows that it only needs to apply it for a specific scenario, it still downloads the stylesheet, but doesn't render block. By separating out the CSS into multiple files, the main render-blocking file, in this case <code>styles.css</code>, is much smaller, reducing the time that rendering is blocked.</p></div></section><section aria-labelledby="improving_font_performance"><h2 id="improving_font_performance"><a href="#improving_font_performance">Improving font performance</a></h2><div class="section-content"><p>This section contains some useful tips for improving web font performance.</p> <p>In general, think carefully about the fonts you use on your site. Some font files can be very large (multiple megabytes). While it can be tempting to use lots of fonts for visual excitement, this can slow down page load significantly, and cause your site to look like a mess. You probably only need about two or three fonts, and you can get away with less if you choose to use <a href="/en-US/docs/Learn/CSS/Styling_text/Fundamentals#web_safe_fonts">web safe fonts</a>.</p></div></section><section aria-labelledby="font_loading"><h3 id="font_loading"><a href="#font_loading">Font loading</a></h3><div class="section-content"><p>Bear in mind that a font is only loaded when it is actually applied to an element using the <a href="/en-US/docs/Web/CSS/font-family"><code>font-family</code></a> property, not when it is first referenced using the <a href="/en-US/docs/Web/CSS/@font-face"><code>@font-face</code></a> at-rule:</p> <div class="code-example"><div class="example-header"><span class="language-name">css</span></div><pre class="brush: css notranslate"><code>/* Font not loaded here */ @font-face { font-family: "Open Sans"; src: url("OpenSans-Regular-webfont.woff2") format("woff2"); } h1, h2, h3 { /* It is actually loaded here */ font-family: "Open Sans"; } </code></pre></div> <p>It can therefore be beneficial to use <code>rel="preload"</code> to load important fonts early, so they will be available more quickly when they are actually needed:</p> <div class="code-example"><div class="example-header"><span class="language-name">html</span></div><pre class="brush: html notranslate"><code><link rel="preload" href="OpenSans-Regular-webfont.woff2" as="font" type="font/woff2" crossorigin /> </code></pre></div> <p>This is more likely to be beneficial if your <code>font-family</code> declaration is hidden inside a large external stylesheet, and won't be reached until significantly later in the parsing process. It is a tradeoff however — font files are quite large, and if you preload too many of them, you may delay other resources.</p> <p>You can also consider:</p> <ul> <li>Using <a href="/en-US/docs/Web/HTML/Attributes/rel/preconnect"><code>rel="preconnect"</code></a> to make an early connection with the font provider. See <a href="https://web.dev/articles/font-best-practices#preconnect_to_critical_third-party_origins" class="external" target="_blank">Preconnect to critical third-party origins</a> for details.</li> <li>Using the <a href="/en-US/docs/Web/API/CSS_Font_Loading_API">CSS Font Loading API</a> to customize the font loading behavior via JavaScript.</li> </ul></div></section><section aria-labelledby="loading_only_the_glyphs_you_need"><h3 id="loading_only_the_glyphs_you_need"><a href="#loading_only_the_glyphs_you_need">Loading only the glyphs you need</a></h3><div class="section-content"><p>When choosing a font for body copy, it is harder to be sure of the glyphs that will be used in it, especially if you are dealing with user-generated content and/or content across multiple languages.</p> <p>However, if you know you are going to use a specific set of glyphs (for example, glyphs for headings or specific punctuation characters only), you could limit the number of glyphs the browser has to download. This can be done by creating a font file that only contains the required subset. A process called <a href="https://fonts.google.com/knowledge/glossary/subsetting" class="external" target="_blank">subsetting</a>. The <a href="/en-US/docs/Web/CSS/@font-face/unicode-range"><code>unicode-range</code></a> <code>@font-face</code> descriptor can then be used to specify when your subset font is used. If the page doesn't use any character in this range, the font is not downloaded.</p> <div class="code-example"><div class="example-header"><span class="language-name">css</span></div><pre class="brush: css notranslate"><code>@font-face { font-family: "Open Sans"; src: url("OpenSans-Regular-webfont.woff2") format("woff2"); unicode-range: U+0025-00FF; } </code></pre></div></div></section><section aria-labelledby="defining_font_display_behavior_with_the_font-display_descriptor"><h3 id="defining_font_display_behavior_with_the_font-display_descriptor"><a href="#defining_font_display_behavior_with_the_font-display_descriptor">Defining font display behavior with the <code>font-display</code> descriptor</a></h3><div class="section-content"><p>Applied to the <code>@font-face</code> at-rule, the <a href="/en-US/docs/Web/CSS/@font-face/font-display"><code>font-display</code></a> descriptor defines how font files are loaded and displayed by the browser, allowing text to appear with a fallback font while a font loads, or fails to load. This improves performance by making the text visible instead of having a blank screen, with a trade-off being a flash of unstyled text.</p> <div class="code-example"><div class="example-header"><span class="language-name">css</span></div><pre class="brush: css notranslate"><code>@font-face { font-family: someFont; src: url(/path/to/fonts/someFont.woff) format("woff"); font-weight: 400; font-style: normal; font-display: fallback; } </code></pre></div></div></section><section aria-labelledby="optimizing_styling_recalculation_with_css_containment"><h2 id="optimizing_styling_recalculation_with_css_containment"><a href="#optimizing_styling_recalculation_with_css_containment">Optimizing styling recalculation with CSS containment</a></h2><div class="section-content"><p>By using the properties defined in the <a href="/en-US/docs/Web/CSS/CSS_containment">CSS containment</a> module, you can instruct the browser to isolate different parts of a page and optimize their rendering independently from one another. This allows for improved performance in rendering individual sections. As an example, you can specify to the browser to not render certain containers until they are visible in the viewport.</p> <p>The <a href="/en-US/docs/Web/CSS/contain"><code>contain</code></a> property allows an author to specify exactly what <a href="/en-US/docs/Web/CSS/CSS_containment/Using_CSS_containment">containment types</a> they want applied to individual containers on the page. This allows the browser to recalculate layout, style, paint, size, or any combination of them for a limited part of the DOM.</p> <div class="code-example"><div class="example-header"><span class="language-name">css</span></div><pre class="brush: css notranslate"><code>article { contain: content; } </code></pre></div> <p>The <a href="/en-US/docs/Web/CSS/content-visibility"><code>content-visibility</code></a> property is a useful shortcut, which allows authors to apply a strong set of containments on a set of containers and specify that the browser should not lay out and render those containers until needed.</p> <p>A second property, <a href="/en-US/docs/Web/CSS/contain-intrinsic-size"><code>contain-intrinsic-size</code></a>, is also available, which allows you to provide a placeholder size for containers while they are under the effects of containment. This means that the containers will take up space even if their contents have not yet been rendered, allowing containment to do its performance magic without the risk of scroll bar shift and jank as elements render and come into view. This improves the quality of the user experience as the content is loaded.</p> <div class="code-example"><div class="example-header"><span class="language-name">css</span></div><pre class="brush: css notranslate"><code>article { content-visibility: auto; contain-intrinsic-size: 1000px; } </code></pre></div><ul class="prev-next"> <li><a class="button secondary" href="/en-US/docs/Learn/Performance/HTML"><span class="button-wrap"> Previous </span></a></li> <li><a class="button secondary" href="/en-US/docs/Learn/Performance"><span class="button-wrap"> Overview: Web performance</span></a></li> <li><a class="button secondary" href="/en-US/docs/Learn/Performance/business_case_for_performance"><span class="button-wrap"> Next </span></a></li> </ul></div></section><section aria-labelledby="see_also"><h2 id="see_also"><a href="#see_also">See also</a></h2><div class="section-content"><ul> <li><a href="/en-US/docs/Web/Performance/CSS_JavaScript_animation_performance">CSS animation performance</a></li> <li><a href="https://web.dev/articles/font-best-practices" class="external" target="_blank">Best practices for fonts</a> on web.dev (2022)</li> <li><a href="https://web.dev/articles/content-visibility" class="external" target="_blank">content-visibility: the new CSS property that boosts your rendering performance</a> on web.dev (2022)</li> </ul></div></section></article><aside class="article-footer"><div class="article-footer-inner"><div class="svg-container"><svg xmlns="http://www.w3.org/2000/svg" width="162" height="162" viewBox="0 0 162 162" fill="none" role="none"><mask id="b" fill="#fff"><path d="M97.203 47.04c8.113-7.886 18.004-13.871 28.906-17.492a78 78 0 0 1 33.969-3.39c11.443 1.39 22.401 5.295 32.024 11.411s17.656 14.28 23.476 23.86c5.819 9.579 9.269 20.318 10.083 31.385a69.85 69.85 0 0 1-5.387 32.44c-4.358 10.272-11.115 19.443-19.747 26.801-8.632 7.359-18.908 12.709-30.034 15.637l-6.17-21.698c7.666-2.017 14.746-5.703 20.694-10.773 5.948-5.071 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Metadata in HTML</a></li><li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/HTML_text_fundamentals\">HTML text fundamentals</a></li><li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Creating_hyperlinks\">Creating hyperlinks</a></li><li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Advanced_text_formatting\">Advanced text formatting</a></li><li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Document_and_website_structure\">Document and website structure</a></li><li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Debugging_HTML\">Debugging HTML</a></li><li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Marking_up_a_letter\">Marking up a letter</a></li><li><a href=\"/en-US/docs/Learn/HTML/Introduction_to_HTML/Structuring_a_page_of_content\">Structuring a page of content</a></li></ol></details></li><li><details><summary>Multimedia and embedding</summary><ol><li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding\">Multimedia and embedding</a></li><li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Images_in_HTML\">Images in HTML</a></li><li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Video_and_audio_content\">Video and audio content</a></li><li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Other_embedding_technologies\">From object to iframe — other embedding technologies</a></li><li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Adding_vector_graphics_to_the_Web\">Adding vector graphics to the web</a></li><li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Responsive_images\">Responsive images</a></li><li><a href=\"/en-US/docs/Learn/HTML/Multimedia_and_embedding/Mozilla_splash_page\">Mozilla splash page</a></li></ol></details></li><li><details><summary>HTML tables</summary><ol><li><a href=\"/en-US/docs/Learn/HTML/Tables\">HTML tables</a></li><li><a href=\"/en-US/docs/Learn/HTML/Tables/Basics\">HTML table basics</a></li><li><a href=\"/en-US/docs/Learn/HTML/Tables/Advanced\">HTML table advanced features and accessibility</a></li><li><a href=\"/en-US/docs/Learn/HTML/Tables/Structuring_planet_data\">Structuring planet data</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/CSS\">CSS — Styling the web</a></li><li><details><summary>CSS first steps</summary><ol><li><a href=\"/en-US/docs/Learn/CSS/First_steps\">CSS first steps</a></li><li><a href=\"/en-US/docs/Learn/CSS/First_steps/What_is_CSS\">What is CSS?</a></li><li><a href=\"/en-US/docs/Learn/CSS/First_steps/Getting_started\">Getting started with CSS</a></li><li><a href=\"/en-US/docs/Learn/CSS/First_steps/How_CSS_is_structured\">How CSS is structured</a></li><li><a href=\"/en-US/docs/Learn/CSS/First_steps/How_CSS_works\">How CSS works</a></li><li><a href=\"/en-US/docs/Learn/CSS/First_steps/Styling_a_biography_page\">Styling a biography page</a></li></ol></details></li><li><details><summary>CSS building blocks</summary><ol><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks\">CSS building blocks</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Selectors\">CSS selectors</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Selectors/Type_Class_and_ID_Selectors\">Type, class, and ID selectors</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Selectors/Attribute_selectors\">Attribute selectors</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Selectors/Pseudo-classes_and_pseudo-elements\">Pseudo-classes and pseudo-elements</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Selectors/Combinators\">Combinators</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Cascade_and_inheritance\">Cascade, specificity, and inheritance</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Cascade_layers\">Cascade layers</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/The_box_model\">The box model</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Backgrounds_and_borders\">Backgrounds and borders</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Handling_different_text_directions\">Handling different text directions</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Overflowing_content\">Overflowing content</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Values_and_units\">CSS values and units</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Sizing_items_in_CSS\">Sizing items in CSS</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Images_media_form_elements\">Images, media, and form elements</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Styling_tables\">Styling tables</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Advanced_styling_effects\">Advanced styling effects</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Debugging_CSS\">Debugging CSS</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Organizing\">Organizing your CSS</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Fundamental_CSS_comprehension\">Fundamental CSS comprehension</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/Creating_fancy_letterheaded_paper\">Creating fancy letterheaded paper</a></li><li><a href=\"/en-US/docs/Learn/CSS/Building_blocks/A_cool_looking_box\">A cool-looking box</a></li></ol></details></li><li><details><summary>Styling text</summary><ol><li><a href=\"/en-US/docs/Learn/CSS/Styling_text\">CSS styling text</a></li><li><a href=\"/en-US/docs/Learn/CSS/Styling_text/Fundamentals\">Fundamental text and font styling</a></li><li><a href=\"/en-US/docs/Learn/CSS/Styling_text/Styling_lists\">Styling lists</a></li><li><a href=\"/en-US/docs/Learn/CSS/Styling_text/Styling_links\">Styling links</a></li><li><a href=\"/en-US/docs/Learn/CSS/Styling_text/Web_fonts\">Web fonts</a></li><li><a href=\"/en-US/docs/Learn/CSS/Styling_text/Typesetting_a_homepage\">Typesetting a community school homepage</a></li></ol></details></li><li><details><summary>CSS layout</summary><ol><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout\">CSS layout</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Introduction\">Introduction to CSS layout</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Normal_Flow\">Normal Flow</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Flexbox\">Flexbox</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Grids\">Grids</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Floats\">Floats</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Positioning\">Positioning</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Multiple-column_Layout\">Multiple-column layout</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Responsive_Design\">Responsive design</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Media_queries\">Beginner's guide to media queries</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Legacy_Layout_Methods\">Legacy layout methods</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Supporting_Older_Browsers\">Supporting older browsers</a></li><li><a href=\"/en-US/docs/Learn/CSS/CSS_layout/Fundamental_Layout_Comprehension\">Fundamental layout comprehension</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/JavaScript\">JavaScript — Dynamic client-side scripting</a></li><li><details><summary>JavaScript first steps</summary><ol><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps\">JavaScript first steps</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/What_is_JavaScript\">What is JavaScript?</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/A_first_splash\">A first splash into JavaScript</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/What_went_wrong\">What went wrong? Troubleshooting JavaScript</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/Variables\">Storing the information you need — Variables</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/Math\">Basic math in JavaScript — numbers and operators</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/Strings\">Handling text — strings in JavaScript</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/Useful_string_methods\">Useful string methods</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/Arrays\">Arrays</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/First_steps/Silly_story_generator\">Silly story generator</a></li></ol></details></li><li><details><summary>JavaScript building blocks</summary><ol><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks\">JavaScript building blocks</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks/conditionals\">Making decisions in your code — conditionals</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks/Looping_code\">Looping code</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks/Functions\">Functions — reusable blocks of code</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks/Build_your_own_function\">Build your own function</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks/Return_values\">Function return values</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks/Events\">Introduction to events</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks/Event_bubbling\">Event bubbling</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Building_blocks/Image_gallery\">Image gallery</a></li></ol></details></li><li><details><summary>Introducing JavaScript objects</summary><ol><li><a href=\"/en-US/docs/Learn/JavaScript/Objects\">Introducing JavaScript objects</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Objects/Basics\">JavaScript object basics</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Objects/Object_prototypes\">Object prototypes</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Objects/Object-oriented_programming\">Object-oriented programming</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Objects/Classes_in_JavaScript\">Classes in JavaScript</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Objects/JSON\">Working with JSON</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Objects/Object_building_practice\">Object building practice</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Objects/Adding_bouncing_balls_features\">Adding features to our bouncing balls demo</a></li></ol></details></li><li><details><summary>Asynchronous JavaScript</summary><ol><li><a href=\"/en-US/docs/Learn/JavaScript/Asynchronous\">Asynchronous JavaScript</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Asynchronous/Introducing\">Introducing asynchronous JavaScript</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Asynchronous/Promises\">How to use promises</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Asynchronous/Implementing_a_promise-based_API\">How to implement a promise-based API</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Asynchronous/Introducing_workers\">Introducing workers</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Asynchronous/Sequencing_animations\">Sequencing animations</a></li></ol></details></li><li><details><summary>Client-side web APIs</summary><ol><li><a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs\">Client-side web APIs</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Introduction\">Introduction to web APIs</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Manipulating_documents\">Manipulating documents</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Fetching_data\">Fetching data from the server</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Third_party_APIs\">Third-party APIs</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Drawing_graphics\">Drawing graphics</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Video_and_audio_APIs\">Video and Audio APIs</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Client-side_storage\">Client-side storage</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/Forms\">Web forms — Working with user data</a></li><li><details><summary>Web form building blocks</summary><ol><li><a href=\"/en-US/docs/Learn/Forms\">Web form building blocks</a></li><li><a href=\"/en-US/docs/Learn/Forms/Your_first_form\">Your first form</a></li><li><a href=\"/en-US/docs/Learn/Forms/How_to_structure_a_web_form\">How to structure a web form</a></li><li><a href=\"/en-US/docs/Learn/Forms/Basic_native_form_controls\">Basic native form controls</a></li><li><a href=\"/en-US/docs/Learn/Forms/HTML5_input_types\">The HTML5 input types</a></li><li><a href=\"/en-US/docs/Learn/Forms/Other_form_controls\">Other form controls</a></li><li><a href=\"/en-US/docs/Learn/Forms/Styling_web_forms\">Styling web forms</a></li><li><a href=\"/en-US/docs/Learn/Forms/Advanced_form_styling\">Advanced form styling</a></li><li><a href=\"/en-US/docs/Learn/Forms/UI_pseudo-classes\">UI pseudo-classes</a></li><li><a href=\"/en-US/docs/Learn/Forms/Form_validation\">Client-side form validation</a></li><li><a href=\"/en-US/docs/Learn/Forms/Sending_and_retrieving_form_data\">Sending form data</a></li></ol></details></li><li><details><summary>Advanced web form techniques</summary><ol><li><a href=\"/en-US/docs/Learn/Forms/How_to_build_custom_form_controls\">How to build custom form controls</a></li><li><a href=\"/en-US/docs/Learn/Forms/Sending_forms_through_JavaScript\">Sending forms through JavaScript</a></li><li><a href=\"/en-US/docs/Learn/Forms/Property_compatibility_table_for_form_controls\">CSS property compatibility table for form controls</a></li><li><a href=\"/en-US/docs/Learn/Forms/HTML_forms_in_legacy_browsers\">HTML forms in legacy browsers</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/Accessibility\">Accessibility — Make the web usable by everyone</a></li><li><details><summary>Accessibility guides</summary><ol><li><a href=\"/en-US/docs/Learn/Accessibility\">Accessibility</a></li><li><a href=\"/en-US/docs/Learn/Accessibility/What_is_accessibility\">What is accessibility?</a></li><li><a href=\"/en-US/docs/Learn/Accessibility/HTML\">HTML: A good basis for accessibility</a></li><li><a href=\"/en-US/docs/Learn/Accessibility/CSS_and_JavaScript\">CSS and JavaScript accessibility best practices</a></li><li><a href=\"/en-US/docs/Learn/Accessibility/WAI-ARIA_basics\">WAI-ARIA basics</a></li><li><a href=\"/en-US/docs/Learn/Accessibility/Multimedia\">Accessible multimedia</a></li><li><a href=\"/en-US/docs/Learn/Accessibility/Mobile\">Mobile accessibility</a></li><li><a href=\"/en-US/docs/Learn/Accessibility/Accessibility_troubleshooting\">Assessment: Accessibility troubleshooting</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/Performance\">Performance — Making websites fast and responsive</a></li><li><details open=\"\"><summary>Performance guides</summary><ol><li><a href=\"/en-US/docs/Learn/Performance\">Web performance</a></li><li><a href=\"/en-US/docs/Learn/Performance/why_web_performance\">The \"why\" of web performance</a></li><li><a href=\"/en-US/docs/Learn/Performance/What_is_web_performance\">What is web performance?</a></li><li><a href=\"/en-US/docs/Learn/Performance/Perceived_performance\">Perceived performance</a></li><li><a href=\"/en-US/docs/Learn/Performance/Measuring_performance\">Measuring performance</a></li><li><a href=\"/en-US/docs/Learn/Performance/Multimedia\">Multimedia: Images</a></li><li><a href=\"/en-US/docs/Learn/Performance/video\">Multimedia: video</a></li><li><a href=\"/en-US/docs/Learn/Performance/JavaScript\">JavaScript performance optimization</a></li><li><a href=\"/en-US/docs/Learn/Performance/HTML\">HTML performance optimization</a></li><li><em><a href=\"/en-US/docs/Learn/Performance/CSS\" aria-current=\"page\">CSS performance optimization</a></em></li><li><a href=\"/en-US/docs/Learn/Performance/business_case_for_performance\">The business case for web performance</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/MathML\">MathML — Writing mathematics with MathML</a></li><li><details><summary>MathML first steps</summary><ol><li><a href=\"/en-US/docs/Learn/MathML/First_steps\">MathML first steps</a></li><li><a href=\"/en-US/docs/Learn/MathML/First_steps/Getting_started\">Getting started with MathML</a></li><li><a href=\"/en-US/docs/Learn/MathML/First_steps/Text_containers\">MathML Text Containers</a></li><li><a href=\"/en-US/docs/Learn/MathML/First_steps/Fractions_and_roots\">MathML fractions and roots</a></li><li><a href=\"/en-US/docs/Learn/MathML/First_steps/Scripts\">MathML scripted elements</a></li><li><a href=\"/en-US/docs/Learn/MathML/First_steps/Tables\">MathML tables</a></li><li><a href=\"/en-US/docs/Learn/MathML/First_steps/Three_famous_mathematical_formulas\">Three famous mathematical formulas</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/../Games\">Games — Developing games for the web</a></li><li><details><summary>Guides and tutorials</summary><ol><li><a href=\"/en-US/docs/Games/Introduction\">Introduction to game development for the Web</a></li><li><a href=\"/en-US/docs/Games/Techniques\">Techniques for game development</a></li><li><a href=\"/en-US/docs/Games/Tutorials\">Tutorials</a></li><li><a href=\"/en-US/docs/Games/Publishing_games\">Publishing games</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/Tools_and_testing\">Tools and testing</a></li><li><details><summary>Client-side web development tools</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools\">Understanding client-side web development tools</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Overview\">Client-side tooling overview</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Command_line\">Command line crash course</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Package_management\">Package management basics</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Introducing_complete_toolchain\">Introducing a complete toolchain</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Understanding_client-side_tools/Deployment\">Deploying our app</a></li></ol></details></li><li><details><summary>Introduction to client-side frameworks</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Introduction\">Introduction to client-side frameworks</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Main_features\">Framework main features</a></li></ol></details></li><li><details><summary>React</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_getting_started\">Getting started with React</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_todo_list_beginning\">Beginning our React todo list</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_components\">Componentizing our React app</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_interactivity_events_state\">React interactivity: Events and state</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_interactivity_filtering_conditional_rendering\">React interactivity: Editing, filtering, conditional rendering</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_accessibility\">Accessibility in React</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/React_resources\">React resources</a></li></ol></details></li><li><details><summary>Ember</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_getting_started\">Getting started with Ember</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_structure_componentization\">Ember app structure and componentization</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_interactivity_events_state\">Ember interactivity: Events, classes and state</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_conditional_footer\">Ember Interactivity: Footer functionality, conditional rendering</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_routing\">Routing in Ember</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Ember_resources\">Ember resources and troubleshooting</a></li></ol></details></li><li><details><summary>Vue</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_getting_started\">Getting started with Vue</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_first_component\">Creating our first Vue component</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_rendering_lists\">Rendering a list of Vue components</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_methods_events_models\">Adding a new todo form: Vue events, methods, and models</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_styling\">Styling Vue components with CSS</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_computed_properties\">Using Vue computed properties</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_conditional_rendering\">Vue conditional rendering: editing existing todos</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_refs_focus_management\">Vue refs and lifecycle methods for focus management</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Vue_resources\">Vue resources</a></li></ol></details></li><li><details><summary>Svelte</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_getting_started\">Getting started with Svelte</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_Todo_list_beginning\">Starting our Svelte to-do list app</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_variables_props\">Dynamic behavior in Svelte: working with variables and props</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_components\">Componentizing our Svelte app</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_reactivity_lifecycle_accessibility\">Advanced Svelte: Reactivity, lifecycle, accessibility</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_stores\">Working with Svelte stores</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_TypeScript\">TypeScript support in Svelte</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Svelte_deployment_next\">Deployment and next steps</a></li></ol></details></li><li><details><summary>Angular</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_getting_started\">Getting started with Angular</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_todo_list_beginning\">Beginning our Angular todo list app</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_styling\">Styling our Angular app</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_item_component\">Creating an item component</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_filtering\">Filtering our to-do items</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Client-side_JavaScript_frameworks/Angular_building\">Building Angular applications and further resources</a></li></ol></details></li><li><details><summary>Git and GitHub</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/GitHub\">Git and GitHub</a></li></ol></details></li><li><details><summary>Cross browser testing</summary><ol><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing\">Cross browser testing</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Introduction\">Introduction to cross-browser testing</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Testing_strategies\">Strategies for carrying out testing</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/HTML_and_CSS\">Handling common HTML and CSS problems</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/JavaScript\">Handling common JavaScript problems</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Accessibility\">Handling common accessibility problems</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Feature_detection\">Implementing feature detection</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Automated_testing\">Introduction to automated testing</a></li><li><a href=\"/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Your_own_automation_environment\">Setting up your own test automation environment</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/Server-side\">Server-side website programming</a></li><li><details><summary>First steps</summary><ol><li><a href=\"/en-US/docs/Learn/Server-side/First_steps\">Server-side website programming first steps</a></li><li><a href=\"/en-US/docs/Learn/Server-side/First_steps/Introduction\">Introduction to the server side</a></li><li><a href=\"/en-US/docs/Learn/Server-side/First_steps/Client-Server_overview\">Client-Server Overview</a></li><li><a href=\"/en-US/docs/Learn/Server-side/First_steps/Web_frameworks\">Server-side web frameworks</a></li><li><a href=\"/en-US/docs/Learn/Server-side/First_steps/Website_security\">Website security</a></li></ol></details></li><li><details><summary>Django web framework (Python)</summary><ol><li><a href=\"/en-US/docs/Learn/Server-side/Django\">Django Web Framework (Python)</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Introduction\">Django introduction</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/development_environment\">Setting up a Django development environment</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Tutorial_local_library_website\">Django Tutorial: The Local Library website</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/skeleton_website\">Django Tutorial Part 2: Creating a skeleton website</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Models\">Django Tutorial Part 3: Using models</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Admin_site\">Django Tutorial Part 4: Django admin site</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Home_page\">Django Tutorial Part 5: Creating our home page</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Generic_views\">Django Tutorial Part 6: Generic list and detail views</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Sessions\">Django Tutorial Part 7: Sessions framework</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Authentication\">Django Tutorial Part 8: User authentication and permissions</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Forms\">Django Tutorial Part 9: Working with forms</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Testing\">Django Tutorial Part 10: Testing a Django web application</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/Deployment\">Django Tutorial Part 11: Deploying Django to production</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/web_application_security\">Django web application security</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Django/django_assessment_blog\">Assessment: DIY Django mini blog</a></li></ol></details></li><li><details><summary>Express Web Framework (Node.js/JavaScript)</summary><ol><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs\">Express web framework (Node.js/JavaScript)</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/Introduction\">Express/Node introduction</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/development_environment\">Setting up a Node development environment</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/Tutorial_local_library_website\">Express Tutorial: The Local Library website</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/skeleton_website\">Express Tutorial Part 2: Creating a skeleton website</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/mongoose\">Express Tutorial Part 3: Using a Database (with Mongoose)</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/routes\">Express Tutorial Part 4: Routes and controllers</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/Displaying_data\">Express Tutorial Part 5: Displaying library data</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/forms\">Express Tutorial Part 6: Working with forms</a></li><li><a href=\"/en-US/docs/Learn/Server-side/Express_Nodejs/deployment\">Express Tutorial Part 7: Deploying to production</a></li></ol></details></li><li class=\"section\"><a href=\"/en-US/docs/Learn/Common_questions\">Further resources</a></li><li><details><summary>Common questions</summary><ol><li><a href=\"/en-US/docs/Learn/Common_questions\">Common questions</a></li><li><a href=\"/en-US/docs/Learn/HTML/Howto\">Use HTML to solve common problems</a></li><li><a href=\"/en-US/docs/Learn/CSS/Howto\">Use CSS to solve common problems</a></li><li><a href=\"/en-US/docs/Learn/JavaScript/Howto\">Solve common problems in your JavaScript code</a></li><li><a href=\"/en-US/docs/Learn/Common_questions/Web_mechanics\">Web mechanics</a></li><li><a href=\"/en-US/docs/Learn/Common_questions/Tools_and_setup\">Tools and setup</a></li><li><a href=\"/en-US/docs/Learn/Common_questions/Design_and_accessibility\">Design and accessibility</a></li></ol></details></li></ol>","sidebarMacro":"LearnSidebar","body":[{"type":"prose","value":{"id":null,"title":null,"isH3":false,"content":"<ul class=\"prev-next\">\n <li><a class=\"button secondary\" href=\"/en-US/docs/Learn/Performance/HTML\"><span class=\"button-wrap\"> Previous </span></a></li>\n <li><a class=\"button secondary\" href=\"/en-US/docs/Learn/Performance\"><span class=\"button-wrap\"> Overview: Web performance</span></a></li>\n <li><a class=\"button secondary\" href=\"/en-US/docs/Learn/Performance/business_case_for_performance\"><span class=\"button-wrap\"> Next </span></a></li>\n</ul>\n<p>When developing a website, you need to consider how the browser is handling the CSS on your site. To mitigate any performance issues that CSS might be causing, you should optimize it. For example, you should optimize the CSS to mitigate <a href=\"/en-US/docs/Glossary/Render_blocking\">render-blocking</a> and minimize the number of required reflows. This article walks you through key CSS performance optimization techniques.</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/Getting_started_with_the_web/Installing_basic_software\">Basic software installed</a>, and basic knowledge of\n <a href=\"/en-US/docs/Learn/Getting_started_with_the_web\">client-side web technologies</a>.\n </td>\n </tr>\n <tr>\n <th scope=\"row\">Objective:</th>\n <td>\n To learn about the impact of CSS on website performance\n and how to optimize your CSS to improve performance.\n </td>\n </tr>\n </tbody>\n</table></figure>"}},{"type":"prose","value":{"id":"to_optimize_or_not_to_optimize","title":"To optimize or not to optimize","isH3":false,"content":"<p>The first question you should answer before starting to optimize your CSS is \"what do I need to optimize?\". Some of the tips and techniques discussed below are good practices that will benefit just about any web project, whereas some are only needed in certain situations. Trying to apply all these techniques everywhere is probably unnecessary, and may be a waste of your time. You should figure out what performance optimizations are actually needed in each project.</p>\n<p>To do this, you need to <a href=\"/en-US/docs/Learn/Performance/Measuring_performance\">measure the performance</a> of your site. As the previous link shows, there are several different ways to measure performance, some involving sophisticated <a href=\"/en-US/docs/Web/API/Performance_API\">performance APIs</a>. The best way to get started however, is to learn how to use tools such as built-in browser <a href=\"/en-US/docs/Learn/Performance/Measuring_performance#network_monitor_tools\">network</a> and <a href=\"/en-US/docs/Learn/Performance/Measuring_performance#performance_monitor_tools\">performance</a> tools, to see what parts of the page load are taking a long time and need optimizing.</p>"}},{"type":"prose","value":{"id":"optimizing_rendering","title":"Optimizing rendering","isH3":false,"content":"<p>Browsers follow a specific rendering path — paint only occurs after layout, which occurs after the render tree is created, which in turn requires both the DOM and the CSSOM trees.</p>\n<p>Showing users an unstyled page and then repainting it after the CSS styles have been parsed would be a bad user experience. For this reason, CSS is render blocking until the browser determines that the CSS is required. The browser can paint the page after it has downloaded the CSS and built the <a href=\"/en-US/docs/Glossary/CSSOM\">CSS object model (CSSOM)</a>.</p>\n<p>To optimize the CSSOM construction and improve page performance, you can do one or more of the following based on the current state of your CSS:</p>\n<ul>\n <li>\n <p><strong>Remove unnecessary styles</strong>: This may sound obvious, but it is surprising how many developers forget to clean up unused CSS rules that were added to their stylesheets during development and ended up not being used. All styles get parsed, whether they are being used during layout and painting or not, so it can speed up page rendering to get rid of unused ones. As <a href=\"https://css-tricks.com/how-do-you-remove-unused-css-from-a-site/\" class=\"external\" target=\"_blank\">How Do You Remove Unused CSS From a Site?</a> (csstricks.com, 2019) summarizes, this is a difficult problem to solve for a large codebase, and there isn't a magic bullet to reliably find and remove unused CSS. You need to do the hard work of keeping your CSS modular and being careful and deliberate about what is added and removed.</p>\n </li>\n <li>\n <p><strong>Split CSS into separate modules</strong>: Keeping CSS modular means that CSS not required at page load can be loaded later on, reducing initial CSS render-blocking and loading times. The simplest way to do this is by splitting up your CSS into separate files and loading only what is needed:</p>\n <div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">html</span></div><pre class=\"brush: html notranslate\"><code>\n<link rel=\"stylesheet\" href=\"styles.css\" />\n\n\n<link rel=\"stylesheet\" href=\"print.css\" media=\"print\" />\n\n\n<link\n rel=\"stylesheet\"\n href=\"mobile.css\"\n media=\"screen and (max-width: 480px)\" />\n</code></pre></div>\n <p>The above example provides three sets of styles — default styles that will always load, styles that will only be loaded when the document is being printed, and styles that will be loaded only by devices with narrow screens. By default, the browser assumes that each specified style sheet is render-blocking. You can tell the browser when a style sheet should be applied by adding a <code>media</code> attribute containing a <a href=\"/en-US/docs/Web/CSS/CSS_media_queries/Using_media_queries\">media query</a>. When the browser sees a style sheet that it only needs to apply in a specific scenario, it still downloads the stylesheet, but doesn't render-block. By separating the CSS into multiple files, the main render-blocking file, in this case <code>styles.css</code>, is much smaller, reducing the time for which rendering is blocked.</p>\n </li>\n <li>\n <p><strong>Minify and compress your CSS</strong>: Minifying involves removing all the whitespace in the file that is only there for human readability, once the code is put into production. You can reduce loading times considerably by minifying your CSS. Minification is generally done as part of a build process (for example, most JavaScript frameworks will minify code when you build a project ready for deployment). In addition to minification, make sure that the server that your site is hosted on uses compression such as gzip on files before serving them.</p>\n </li>\n <li>\n <p><strong>Simplify selectors</strong>: People often write selectors that are more complex than needed for applying the required styles. This not only increases file sizes, but also the parsing time for those selectors. For example:</p>\n <div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">css</span></div><pre class=\"brush: css notranslate\"><code>/* Very specific selector */\nbody div#main-content article.post h2.headline {\n font-size: 24px;\n}\n\n/* You probably only need this */\n.headline {\n font-size: 24px;\n}\n</code></pre></div>\n <p>Making your selectors less complex and specific is also good for maintenance. It is easy to understand what simple selectors are doing, and it is easy to override styles when needed later on if the selectors are less <a href=\"/en-US/docs/Learn/CSS/Building_blocks/Cascade_and_inheritance#specificity_2\">specific</a>.</p>\n </li>\n <li>\n <p><strong>Don't apply styles to more elements than needed</strong>: A common mistake is to apply styles to all elements using the <a href=\"/en-US/docs/Web/CSS/Universal_selectors\">universal selector</a>, or at least, to more elements than needed. This kind of styling can impact performance negatively, especially on larger sites.</p>\n <div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">css</span></div><pre class=\"brush: css notranslate\"><code>/* Selects every element inside the <body> */\nbody * {\n font-size: 14px;\n display: flex;\n}\n</code></pre></div>\n <p>Remember that many properties (such as <a href=\"/en-US/docs/Web/CSS/font-size\"><code>font-size</code></a>) inherit their values from their parents, so you don't need to apply them everywhere. And powerful tools such as <a href=\"/en-US/docs/Learn/CSS/CSS_layout/Flexbox\">Flexbox</a> need to be used sparingly. Using them everywhere can cause all kinds of unexpected behavior.</p>\n </li>\n <li>\n <p><strong>Cut down on image HTTP requests with CSS sprites</strong>: <a href=\"https://css-tricks.com/css-sprites/\" class=\"external\" target=\"_blank\">CSS sprites</a> is a technique that places several small images (such as icons) that you want to use on your site into a single image file, and then uses different <a href=\"/en-US/docs/Web/CSS/background-position\"><code>background-position</code></a> values to display the chunk of image that you want to show in each different place. This can dramatically cut down on the number of HTTP requests needed to fetch the images.</p>\n </li>\n <li>\n <p><strong>Preload important assets</strong>: You can use <a href=\"/en-US/docs/Web/HTML/Attributes/rel/preload\"><code>rel=\"preload\"</code></a> to turn <a href=\"/en-US/docs/Web/HTML/Element/link\"><code><link></code></a> elements into preloaders for critical assets. This includes CSS files, fonts, and images:</p>\n <div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">html</span></div><pre class=\"brush: html notranslate\"><code><link rel=\"preload\" href=\"style.css\" as=\"style\" />\n\n<link\n rel=\"preload\"\n href=\"ComicSans.woff2\"\n as=\"font\"\n type=\"font/woff2\"\n crossorigin />\n\n<link\n rel=\"preload\"\n href=\"bg-image-wide.png\"\n as=\"image\"\n media=\"(min-width: 601px)\" />\n</code></pre></div>\n <p>With <code>preload</code>, the browser will fetch the referenced resources as soon as possible and make them available in the browser cache so that they will be ready for use sooner when they are referenced in subsequent code. It is useful to preload high-priority resources that the user will encounter early on in a page so that the experience is as smooth as possible. Note how you can also use <code>media</code> attributes to create responsive preloaders.</p>\n <p>See also <a href=\"https://web.dev/articles/preload-critical-assets\" class=\"external\" target=\"_blank\">Preload critical assets to improve loading speed</a> on web.dev (2020)</p>\n </li>\n</ul>"}},{"type":"prose","value":{"id":"handling_animations","title":"Handling animations","isH3":false,"content":"<p>Animations can improve perceived performance, making interfaces feel snappier and making users feel like progress is being made when they are waiting for a page to load (loading spinners, for example). However, larger animations and a higher number of animations will naturally require more processing power to handle, which can degrade performance.</p>\n<p>The simplest advice is to cut down on all unnecessary animations. You could also provide users with a control/site preference to turn off animations if they are using a low-powered device or a mobile device with limited battery power. You could also use JavaScript to control whether or not animation is applied to the page in the first place. There is also a media query called <a href=\"/en-US/docs/Web/CSS/@media/prefers-reduced-motion\"><code>prefers-reduced-motion</code></a> that can be used to selectively serve animation styles or not based on a user's OS-level preferences for animation.</p>\n<p>For essential DOM animations, you are advised to use <a href=\"/en-US/docs/Web/CSS/CSS_animations/Using_CSS_animations\">CSS animations</a> where possible, rather than JavaScript animations (the <a href=\"/en-US/docs/Web/API/Web_Animations_API\">Web Animations API</a> provides a way to directly hook into CSS animations using JavaScript).</p>"}},{"type":"prose","value":{"id":"choosing_properties_to_animate","title":"Choosing properties to animate","isH3":true,"content":"<p>Next, animation performance relies heavily on what properties you are animating. Certain properties, when animated, trigger a <a href=\"/en-US/docs/Glossary/Reflow\">reflow</a> (and therefore also a <a href=\"/en-US/docs/Glossary/Repaint\">repaint</a>) and should be avoided. These include properties that:</p>\n<ul>\n <li>Alter an element's dimensions, such as <a href=\"/en-US/docs/Web/CSS/width\"><code>width</code></a>, <a href=\"/en-US/docs/Web/CSS/height\"><code>height</code></a>, <a href=\"/en-US/docs/Web/CSS/border\"><code>border</code></a>, and <a href=\"/en-US/docs/Web/CSS/padding\"><code>padding</code></a>.</li>\n <li>Reposition an element, such as <a href=\"/en-US/docs/Web/CSS/margin\"><code>margin</code></a>, <a href=\"/en-US/docs/Web/CSS/top\"><code>top</code></a>, <a href=\"/en-US/docs/Web/CSS/bottom\"><code>bottom</code></a>, <a href=\"/en-US/docs/Web/CSS/left\"><code>left</code></a>, and <a href=\"/en-US/docs/Web/CSS/right\"><code>right</code></a>.</li>\n <li>Change an element's layout, such as <a href=\"/en-US/docs/Web/CSS/align-content\"><code>align-content</code></a>, <a href=\"/en-US/docs/Web/CSS/align-items\"><code>align-items</code></a>, and <a href=\"/en-US/docs/Web/CSS/flex\"><code>flex</code></a>.</li>\n <li>Add visual effects that change the element geometry, such as <a href=\"/en-US/docs/Web/CSS/box-shadow\"><code>box-shadow</code></a>.</li>\n</ul>\n<p>Modern browsers are smart enough to repaint only the changed area of the document, rather than the entire page. As a result, larger animations are more costly.</p>\n<p>If at all possible, it is better to animate properties that do not cause reflow/repaint. This includes:</p>\n<ul>\n <li><a href=\"/en-US/docs/Web/CSS/CSS_transforms\">Transforms</a></li>\n <li><a href=\"/en-US/docs/Web/CSS/opacity\"><code>opacity</code></a></li>\n <li><a href=\"/en-US/docs/Web/CSS/filter\"><code>filter</code></a></li>\n</ul>"}},{"type":"prose","value":{"id":"animating_on_the_gpu","title":"Animating on the GPU","isH3":true,"content":"<p>To further improve performance, you should consider moving animation work off the main thread and onto the device's GPU (also referred to as compositing). This is done by choosing specific types of animations that the browser will automatically send to the GPU to handle; these include:</p>\n<ul>\n <li>3D transform animations such as <a href=\"/en-US/docs/Web/CSS/transform\"><code>transform: translateZ()</code></a> and <a href=\"/en-US/docs/Web/CSS/transform-function/rotate3d\"><code>rotate3d()</code></a>.</li>\n <li>Elements with certain other properties animated such as <a href=\"/en-US/docs/Web/CSS/position\"><code>position: fixed</code></a>.</li>\n <li>Elements with <a href=\"/en-US/docs/Web/CSS/will-change\"><code>will-change</code></a> applied (see the section below).</li>\n <li>Certain elements that are rendered in their own layer, including <a href=\"/en-US/docs/Web/HTML/Element/video\"><code><video></code></a>, <a href=\"/en-US/docs/Web/HTML/Element/canvas\"><code><canvas></code></a>, and <a href=\"/en-US/docs/Web/HTML/Element/iframe\"><code><iframe></code></a>.</li>\n</ul>\n<p>Animation on the GPU can result in improved performance, especially on mobile. However, moving animations to GPU is not always that simple. Read <a href=\"https://www.smashingmagazine.com/2016/12/gpu-animation-doing-it-right/\" class=\"external\" target=\"_blank\">CSS GPU Animation: Doing It Right</a> (smashingmagazine.com, 2016) for a very useful and detailed analysis.</p>"}},{"type":"prose","value":{"id":"optimizing_element_changes_with_will-change","title":"Optimizing element changes with <code>will-change</code>","isH3":false,"content":"<p>Browsers may set up optimizations before an element is actually changed. These kinds of optimizations can increase the responsiveness of a page by doing potentially expensive work before it is required. The CSS <a href=\"/en-US/docs/Web/CSS/will-change\"><code>will-change</code></a> property hints to browsers how an element is expected to change.</p>\n<div class=\"notecard note\">\n <p><strong>Note:</strong> <code>will-change</code> is intended to be used as a last resort to try to deal with existing performance problems. It should not be used to anticipate performance problems.</p>\n</div>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">css</span></div><pre class=\"brush: css notranslate\"><code>.element {\n will-change: opacity, transform;\n}\n</code></pre></div>"}},{"type":"prose","value":{"id":"optimizing_for_render_blocking","title":"Optimizing for render blocking","isH3":false,"content":"<p>CSS can scope styles to particular conditions with media queries. Media queries are important for a responsive web design and help us optimize a critical rendering path. The browser blocks rendering until it parses all of these styles but will not block rendering on styles it knows it will not use, such as the print stylesheets. By splitting the CSS into multiple files based on media queries, you can prevent render blocking during download of unused CSS. To create a non-blocking CSS link, move the not-immediately used styles, such as print styles, into separate file, add a <a href=\"/en-US/docs/Web/HTML/Element/link\"><code><link></code></a> to the HTML mark up, and add a media query, in this case stating it's a print stylesheet.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">html</span></div><pre class=\"brush: html notranslate\"><code>\n<link rel=\"stylesheet\" href=\"styles.css\" />\n\n\n<link rel=\"stylesheet\" href=\"print.css\" media=\"print\" />\n\n\n<link\n rel=\"stylesheet\"\n href=\"mobile.css\"\n media=\"screen and (max-width: 480px)\" />\n</code></pre></div>\n<p>By default, the browser assumes that each specified style sheet is render blocking. Tell the browser when the style sheet should be applied by adding a <code>media</code> attribute with the <a href=\"/en-US/docs/Web/CSS/CSS_media_queries/Using_media_queries\">media query</a>. When the browser sees a style sheet it knows that it only needs to apply it for a specific scenario, it still downloads the stylesheet, but doesn't render block. By separating out the CSS into multiple files, the main render-blocking file, in this case <code>styles.css</code>, is much smaller, reducing the time that rendering is blocked.</p>"}},{"type":"prose","value":{"id":"improving_font_performance","title":"Improving font performance","isH3":false,"content":"<p>This section contains some useful tips for improving web font performance.</p>\n<p>In general, think carefully about the fonts you use on your site. Some font files can be very large (multiple megabytes). While it can be tempting to use lots of fonts for visual excitement, this can slow down page load significantly, and cause your site to look like a mess. You probably only need about two or three fonts, and you can get away with less if you choose to use <a href=\"/en-US/docs/Learn/CSS/Styling_text/Fundamentals#web_safe_fonts\">web safe fonts</a>.</p>"}},{"type":"prose","value":{"id":"font_loading","title":"Font loading","isH3":true,"content":"<p>Bear in mind that a font is only loaded when it is actually applied to an element using the <a href=\"/en-US/docs/Web/CSS/font-family\"><code>font-family</code></a> property, not when it is first referenced using the <a href=\"/en-US/docs/Web/CSS/@font-face\"><code>@font-face</code></a> at-rule:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">css</span></div><pre class=\"brush: css notranslate\"><code>/* Font not loaded here */\n@font-face {\n font-family: \"Open Sans\";\n src: url(\"OpenSans-Regular-webfont.woff2\") format(\"woff2\");\n}\n\nh1,\nh2,\nh3 {\n /* It is actually loaded here */\n font-family: \"Open Sans\";\n}\n</code></pre></div>\n<p>It can therefore be beneficial to use <code>rel=\"preload\"</code> to load important fonts early, so they will be available more quickly when they are actually needed:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">html</span></div><pre class=\"brush: html notranslate\"><code><link\n rel=\"preload\"\n href=\"OpenSans-Regular-webfont.woff2\"\n as=\"font\"\n type=\"font/woff2\"\n crossorigin />\n</code></pre></div>\n<p>This is more likely to be beneficial if your <code>font-family</code> declaration is hidden inside a large external stylesheet, and won't be reached until significantly later in the parsing process. It is a tradeoff however — font files are quite large, and if you preload too many of them, you may delay other resources.</p>\n<p>You can also consider:</p>\n<ul>\n <li>Using <a href=\"/en-US/docs/Web/HTML/Attributes/rel/preconnect\"><code>rel=\"preconnect\"</code></a> to make an early connection with the font provider. See <a href=\"https://web.dev/articles/font-best-practices#preconnect_to_critical_third-party_origins\" class=\"external\" target=\"_blank\">Preconnect to critical third-party origins</a> for details.</li>\n <li>Using the <a href=\"/en-US/docs/Web/API/CSS_Font_Loading_API\">CSS Font Loading API</a> to customize the font loading behavior via JavaScript.</li>\n</ul>"}},{"type":"prose","value":{"id":"loading_only_the_glyphs_you_need","title":"Loading only the glyphs you need","isH3":true,"content":"<p>When choosing a font for body copy, it is harder to be sure of the glyphs that will be used in it, especially if you are dealing with user-generated content and/or content across multiple languages.</p>\n<p>However, if you know you are going to use a specific set of glyphs (for example, glyphs for headings or specific punctuation characters only), you could limit the number of glyphs the browser has to download. This can be done by creating a font file that only contains the required subset. A process called <a href=\"https://fonts.google.com/knowledge/glossary/subsetting\" class=\"external\" target=\"_blank\">subsetting</a>. The <a href=\"/en-US/docs/Web/CSS/@font-face/unicode-range\"><code>unicode-range</code></a> <code>@font-face</code> descriptor can then be used to specify when your subset font is used. If the page doesn't use any character in this range, the font is not downloaded.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">css</span></div><pre class=\"brush: css notranslate\"><code>@font-face {\n font-family: \"Open Sans\";\n src: url(\"OpenSans-Regular-webfont.woff2\") format(\"woff2\");\n unicode-range: U+0025-00FF;\n}\n</code></pre></div>"}},{"type":"prose","value":{"id":"defining_font_display_behavior_with_the_font-display_descriptor","title":"Defining font display behavior with the <code>font-display</code> descriptor","isH3":true,"content":"<p>Applied to the <code>@font-face</code> at-rule, the <a href=\"/en-US/docs/Web/CSS/@font-face/font-display\"><code>font-display</code></a> descriptor defines how font files are loaded and displayed by the browser, allowing text to appear with a fallback font while a font loads, or fails to load. This improves performance by making the text visible instead of having a blank screen, with a trade-off being a flash of unstyled text.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">css</span></div><pre class=\"brush: css notranslate\"><code>@font-face {\n font-family: someFont;\n src: url(/path/to/fonts/someFont.woff) format(\"woff\");\n font-weight: 400;\n font-style: normal;\n font-display: fallback;\n}\n</code></pre></div>"}},{"type":"prose","value":{"id":"optimizing_styling_recalculation_with_css_containment","title":"Optimizing styling recalculation with CSS containment","isH3":false,"content":"<p>By using the properties defined in the <a href=\"/en-US/docs/Web/CSS/CSS_containment\">CSS containment</a> module, you can instruct the browser to isolate different parts of a page and optimize their rendering independently from one another. This allows for improved performance in rendering individual sections. As an example, you can specify to the browser to not render certain containers until they are visible in the viewport.</p>\n<p>The <a href=\"/en-US/docs/Web/CSS/contain\"><code>contain</code></a> property allows an author to specify exactly what <a href=\"/en-US/docs/Web/CSS/CSS_containment/Using_CSS_containment\">containment types</a> they want applied to individual containers on the page. This allows the browser to recalculate layout, style, paint, size, or any combination of them for a limited part of the DOM.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">css</span></div><pre class=\"brush: css notranslate\"><code>article {\n contain: content;\n}\n</code></pre></div>\n<p>The <a href=\"/en-US/docs/Web/CSS/content-visibility\"><code>content-visibility</code></a> property is a useful shortcut, which allows authors to apply a strong set of containments on a set of containers and specify that the browser should not lay out and render those containers until needed.</p>\n<p>A second property, <a href=\"/en-US/docs/Web/CSS/contain-intrinsic-size\"><code>contain-intrinsic-size</code></a>, is also available, which allows you to provide a placeholder size for containers while they are under the effects of containment. This means that the containers will take up space even if their contents have not yet been rendered, allowing containment to do its performance magic without the risk of scroll bar shift and jank as elements render and come into view. This improves the quality of the user experience as the content is loaded.</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">css</span></div><pre class=\"brush: css notranslate\"><code>article {\n content-visibility: auto;\n contain-intrinsic-size: 1000px;\n}\n</code></pre></div><ul class=\"prev-next\">\n <li><a class=\"button secondary\" href=\"/en-US/docs/Learn/Performance/HTML\"><span class=\"button-wrap\"> Previous </span></a></li>\n <li><a class=\"button secondary\" href=\"/en-US/docs/Learn/Performance\"><span class=\"button-wrap\"> Overview: Web performance</span></a></li>\n <li><a class=\"button secondary\" href=\"/en-US/docs/Learn/Performance/business_case_for_performance\"><span class=\"button-wrap\"> Next </span></a></li>\n</ul>"}},{"type":"prose","value":{"id":"see_also","title":"See also","isH3":false,"content":"<ul>\n <li><a href=\"/en-US/docs/Web/Performance/CSS_JavaScript_animation_performance\">CSS animation performance</a></li>\n <li><a href=\"https://web.dev/articles/font-best-practices\" class=\"external\" target=\"_blank\">Best practices for fonts</a> on web.dev (2022)</li>\n <li><a href=\"https://web.dev/articles/content-visibility\" class=\"external\" target=\"_blank\">content-visibility: the new CSS property that boosts your rendering performance</a> on web.dev (2022)</li>\n</ul>"}}],"toc":[{"text":"To optimize or not to optimize","id":"to_optimize_or_not_to_optimize"},{"text":"Optimizing rendering","id":"optimizing_rendering"},{"text":"Handling animations","id":"handling_animations"},{"text":"Optimizing element changes with <code>will-change</code>","id":"optimizing_element_changes_with_will-change"},{"text":"Optimizing for render blocking","id":"optimizing_for_render_blocking"},{"text":"Improving font performance","id":"improving_font_performance"},{"text":"Optimizing styling recalculation with CSS containment","id":"optimizing_styling_recalculation_with_css_containment"},{"text":"See also","id":"see_also"}],"summary":"When developing a website, you need to consider how the browser is handling the CSS on your site. To mitigate any performance issues that CSS might be causing, you should optimize it. For example, you should optimize the CSS to mitigate render-blocking and minimize the number of required reflows. This article walks you through key CSS performance optimization techniques.","popularity":0.043,"modified":"2024-02-02T16:58:29.000Z","other_translations":[{"locale":"de","title":"CSS-Leistungsoptimierung","native":"Deutsch"},{"locale":"fr","title":"Optimisation des performances en CSS","native":"Français"},{"locale":"ja","title":"CSS のパフォーマンス最適化","native":"日本語"},{"locale":"zh-CN","title":"CSS 性能优化","native":"中文 (简体)"}],"pageType":"learn-module-chapter","source":{"folder":"en-us/learn/performance/css","github_url":"https://github.com/mdn/content/blob/main/files/en-us/learn/performance/css/index.md","last_commit_url":"https://github.com/mdn/content/commit/4bf03c104b1bca2068dbff927020e7f802c4af7e","filename":"index.md"},"short_title":"CSS performance optimization","parents":[{"uri":"/en-US/docs/Learn","title":"Guides"},{"uri":"/en-US/docs/Learn/Performance","title":"Web performance"},{"uri":"/en-US/docs/Learn/Performance/CSS","title":"CSS performance optimization"}],"pageTitle":"CSS performance optimization - Learn web development | MDN","noIndexing":false}}</script></body></html>