Aims to be the biggest checklist of inclusive design considerations for the web ever. Includes items for accessibility, performance, device support, interoperability, and language. Pull requests welcome!

• Minify CSS and JS, and remove unused/redundant code [#1]
• Maintain terse HTML, without over-reliance on <div> scaffolding [#1]
• Use screen reader and keyboard accessible HTML [#1]
• Compress raster images [#1]
• Optimize SVG path data [#1]
• Make sure heading levels describe a logical section/subsection structure [#1]
• Only include heading elements where they introduce sections of content
• Remove potentially insensitive or uninclusive language (use "singular they") [#1]
• Give video content captions and transcripts
• Provide transcripts for audio content
• Make sure main body (paragraph) text is no smaller than the default (user agent) size [#1]
• Support "pinch zoom" (remove user-scalable=no if present)
• Use relative units (em, rem, and ch), especially for font metrics
• Make sure styles and scripts are not render blocking [#1]
• Install a service worker and cache all applicable assets [#1]
• Use content-based, not device-specific, media queries [#1]
• Provide alternatives and/or descriptions for complex visualizations
• Include only clear, meaningful animations
• Honor requests to remove animation via the prefers-reduced-motion media query
• Make sure controls do not elicit unexpected or jarring behavior [#1] [#2]
• Do not include third parties that compromise user privacy [#1]
• Do not recreate supported and expected browser behaviors with bespoke scripts
• Support Windows high contrast mode (use images, not background images) [#1]
• Provide alternative text for salient images [#1]
• Apply alt=" or aria-hidden="true" to decorative images [#1]
• Make sure text and background colors contrast sufficiently [#1]
• Provide <title>s that name the site and the specific page [#1]
• Provide large touch "targets" for interactive elements [#1]
• Use data tables (<table>) for data only, not visual layout purposes
• Make scrollable elements focusable for keyboard users
• Do not rely on color for differentiation of visual elements
• Use the same design patterns to solve the same problems
• Ensure keyboard focus order is logical regarding visual layout
• Lazy load large image assets
• Honour DNT (Do Not Track) header [#1]
• Translate / spell out acronyms the first time you use them
• Do not hijack standard scrolling behavior
• Move focus between dialogs and the controls that invoked them
• Give all form elements permanently visible labels
• Give grouped form elements group labels
• Place labels above form elements
• Provide status and error messages as WAI-ARIA live regions
• Provide clear, unambiguous focus styles
• Employ well-balanced, highly legible fonts (not too complex or elaborate)
• Do not use very thin font faces [#1]
• Ensure states (pressed, expanded, invalid, etc) are communicated to assistive software
• Match semantics to behavior for assistive technology users
• Provide a default language and use lang="[ISO code]" for subsections in different languages
• Make controls look like controls; give them strong perceived affordance
• Underline links — at least in body copy
• Make sure all content belongs to a landmark element (<header>, <footer>, <nav>, <main>, etc)
• Avoid pure white or pure black shades
• Mark invalid fields clearly and provide associated error messages
• Ensure content is not obscured through zooming (no fixed widths)
• Provide a manifest.json file for identifiable homescreen entries
• Indicate swipe gesture support clearly, and provide simple tap-based alternatives
• Make sure data tables wider than their container can be scrolled horizontally
• Avoid time constraints where possible; provide a clear warning and option to extend where not possible
• Label and describe the same things with the same terminology
• Ensure disabled controls are not focusable
• Do not instate "infinite scroll" by default; provide buttons to load more items
• Avoid justified body text [#1]
• Provide enough spacing between lines of text (line-height) [#1]
• Ensure PDF content is accessible (include tags) [#1]
• Provide a skip link if necessary [#1]
• Avoid all-caps text [#1]
• Ensure that content is written as clearly and simply as possible [#1]
• Provide descriptive captions for figures
• Warn users of links that have unusual behaviors, like linking off-site, or loading a new tab
• Make content easier to find and improve search results with structured data [#1]
• Use textual labels to make voice activation cues obvious
• Do not mark up subheadings/straplines with separate heading elements
• Ensure primary calls to action are easy to recognize and reach
• Avoid images of text — text that cannot be translated, selected, or understood by assistive tech
• Provide a print stylesheet (single column, with interactive content hidden)
• Use well-established, therefore recognizable, icons and symbols
• Subset fonts to just the characters needed
• Instead of obstructing users with CAPTCHAs, use honeypots [#1]
• Begin long, multi-section documents with a table of contents
• Don"t make users perform actions to reveal content unless completely necessary
• If content is meant to be hidden, ensure it is properly hidden to all users
• Make sure controls within hidden content are not focusable
• Use srcset to tailor images to devices and reduce bandwidth costs
• Do not auto focus form fields, on page load
• Break up long and complex forms into discrete sections and/or screens
• Make forms as short as possible; offer shortcuts like autocompleting the address using the postcode
• Ensure the same content is available across different devices and platforms
• Inform the user when there are important changes to the application state
• Make sure the purpose of a link is clearly described: "read more" vs. "read more about accessibility"