Accessibility Glossary

Every meaningful image needs a text equivalent describing its purpose — not its appearance. Decorative images get alt="". 54% of homepages have images missing alt text.

Videos must describe important visual content in the audio track for blind users. Required when dialogue alone doesn't convey what's happening on screen.

Pre-recorded video needs synchronized captions. Pre-recorded audio needs a transcript. Live video needs real-time captions (Level AA). Benefits 430M people with hearing loss plus anyone in a noisy environment.

Text must meet a 4.5:1 contrast ratio against its background (3:1 for large text ≥18pt/14pt bold). The single most common WCAG failure — found on 84% of homepages in the 2026 WebAIM Million.

One <h1> per page. Headings in logical order without skipping levels (h1→h2→h3, never h1→h3). Screen reader users navigate by heading — 67.5% use headings to find information.

Don't use images of text — use real text styled with CSS. Images of text can't be resized, reflowed, recolored, or read by screen readers. Exception for logos.

Structure conveyed visually (headings, lists, tables, form groups) must also be conveyed programmatically. If it looks like a heading, it must be a heading element.

Semantic containers: <header>, <nav>, <main>, <aside>, <footer>. Screen reader users jump between landmarks. Every page needs exactly one <main>.

UI components and graphical objects need at least 3:1 contrast. Applies to icons, form borders, chart segments, focus indicators — anything meaningful that isn't text.

Content must reflow to a single column at 320px wide (400% zoom) without horizontal scrolling. Exceptions for data tables, toolbars, and maps.

Instructions can't depend on shape, size, position, or sound alone. "Press the round green button on the right" fails for anyone who can't see it — always add a text label: "Press Submit".

No loss of content when users override line height to 1.5×, paragraph spacing to 2×, letter spacing to 0.12em, and word spacing to 0.16em. Tests low-vision user overrides.

Color must not be the only visual means of conveying information. Add icons, patterns, underlines, or text labels alongside color coding. Affects 300M people with color vision deficiency.