Semantic HTML — semantic-html.md: Landmarks, headings, images, tables, lists, language markup
ARIA — aria.md: Roles, states, properties, naming, live regions, common mistakes
Keyboard — keyboard.md: Focus order, visibility, roving tabindex, focus traps, restoration
Forms — forms.md: Labels, grouping, required fields, validation, autocomplete
Component patterns — component-patterns.md: Dialog, tabs, accordion, disclosure, menu, combobox, tooltip
WCAG — wcag.md: Full WCAG 2.2 AA criteria tables and compliance checklist

1. Semantic HTML

Use the right element for the right job. Native elements provide built-in keyboard support, screen reader announcements, and focus management that ARIA can only approximate.

Landmarks

Every page must have exactly one <main>.
<header>/<footer> map to banner/contentinfo only as direct children of <body> -- nested inside <article>, <section>, etc. they lose their landmark role.
Label multiple <nav> elements with aria-label or aria-labelledby.
Never duplicate implicit roles (<main role="main">, <nav role="navigation">).
Include all perceivable content within a landmark region.

For the full landmark-to-role mapping table, see semantic-html.md.

Headings

One <h1> per page identifying the primary content.
Never skip heading levels -- <h1> then <h3> breaks the outline.
Use headings for structure, not visual styling.
Every <section> and major content area should begin with a heading.

Interactive Elements

Use <button> for actions (submit, toggle, open dialog) -- activates via Enter and Space. Use <a href> for navigation -- activates via Enter. Never <a href="#" onclick="..."> for actions. Never <button> for navigation.

Tables

Use <th> with scope="col" or scope="row" for headers.
Add <caption> to describe the table's purpose.
Never use tables for layout.

Lists

Use <ul> for unordered, <ol> for ordered, <dl>/<dt>/<dd> for term/description pairs. Screen readers announce list type and item count.

Images

Every <img> must have an alt attribute -- even if empty.
Do not start alt text with "Image of" or "Picture of".
Keep alt text under ~125 characters.
For decorative images, prefer CSS background-image over <img alt="">.
Use real text, not images of text (WCAG 1.4.5).

For alt treatment by image type, see semantic-html.md.

Language and Page Metadata

Set <html lang="..."> on every page (WCAG 3.1.1).
Mark inline language changes: <span lang="fr">bonjour</span> (WCAG 3.1.2).
Expand abbreviations on first use with <abbr>.
Provide a descriptive <title> on every page (WCAG 2.4.2).
Provide a skip link as the first focusable element (WCAG 2.4.1).

2. ARIA

No ARIA is better than bad ARIA. ARIA modifies only the accessibility tree -- it does not change behavior, keyboard interaction, or appearance.

The Five Rules of ARIA

Use native HTML first. If a semantic HTML element exists, use it.
Do not change native semantics unless absolutely necessary. Never <h2 role="tab"> -- use <div role="tab"><h2>...</h2></div>.
All interactive ARIA controls must be keyboard operable. A role="button" must respond to Enter and Space.
Never role="presentation" or aria-hidden="true" on focusable elements.
All interactive elements must have an accessible name.

Naming and Describing

Priority: aria-labelledby > aria-label > <label> > element content

title.

Prefer visible labels over aria-label.
Use aria-labelledby to compose names from multiple elements.
aria-label and aria-labelledby replace native label text, not supplement.
Never aria-label on elements with role="presentation" or role="none".
Never aria-label on <div> without a role -- ignored by most AT.
Use aria-describedby or aria-description for supplementary info after the name is established.

Live Regions

Use aria-live="polite" for non-urgent updates.
Use aria-live="assertive" sparingly -- only errors, alerts, urgent info.
Set live regions in the DOM before content changes. Adding aria-live and content simultaneously may not be announced.
Inject alert content into an existing role="alert" container for reliable announcement.

Place aria-expanded on the trigger element, not the panel.

For widget roles, state/relationship attribute tables, and common ARIA mistakes, see aria.md.

3. Keyboard Navigation

All interactive functionality must be operable with a keyboard alone.

Fundamental Keys

Tab/Shift+Tab between components. Arrow keys within composite widgets. Enter activates links, buttons, menu items. Space activates buttons, checkboxes, toggles. Escape closes overlays.

Focus Order

Never tabindex > 0. Rearrange DOM order instead.
tabindex="0" makes non-interactive elements focusable (use sparingly).
tabindex="-1" for programmatic focus only (dialog containers, skip targets).
Source order = visual order = focus order. CSS reordering (flex-direction: row-reverse, order, grid) must not break this.

Focus Visibility

Never outline: none without a custom focus style replacement.
Use :focus-visible for keyboard-only focus styles.
Focus indicator: at least 3:1 contrast against adjacent background.
Focus indicator area: at least 2px border equivalent (WCAG 2.4.13).
Focused element must not be entirely obscured (WCAG 2.4.11).

Focus Management

Use roving tabindex for composite widgets (tabs, toolbars, menus): active child gets tabindex="0", others get tabindex="-1". Use aria-activedescendant when the container must maintain focus (combobox).

Focus Trap (Dialogs)

On open: focus first focusable element inside dialog.
Trap Tab/Shift+Tab -- wrap last-to-first and first-to-last.
Escape closes dialog.
On close: return focus to the trigger element.
Set aria-modal="true".

Focus Restoration

Deleted item -- focus next item, or previous if last was deleted.
Closed overlay -- focus the trigger that opened it.
Removed section -- focus nearest logical container or heading.
Never let focus fall to document.body.

Disabled Elements

Remove disabled standalone controls from tab sequence (disabled attribute or tabindex="-1").
Keep disabled items focusable inside composite widgets (menus, tabs, trees, listboxes) so screen reader users can discover them.
Use aria-disabled="true" to keep element focusable but not operable.

4. Accessible Forms

Labels

Every <input>, <select>, <textarea> must have a programmatic label.

Priority: <label for/id> > wrapping <label> > aria-labelledby > aria-label.

placeholder is not a label substitute -- disappears on input, unreliable in screen readers.
Visible label text must be contained in the accessible name (WCAG 2.5.3).

Grouping

Group related controls with <fieldset> + <legend>. Required for: radio groups, checkbox groups, related input sets (address, date parts).
Use role="group" with aria-labelledby when <fieldset> is impractical.

Required Fields

Use required attribute or aria-required="true".
Indicate visually with asterisk + "* Required" legend.
Never color alone for required status.

Validation and Errors

Display errors adjacent to the invalid field.
Associate errors via aria-describedby or aria-errormessage.
Set aria-invalid="true" on invalid fields.
Use role="alert" on error container for immediate announcement.
For multiple errors: error summary at top with links to fields; move focus to summary on validation failure.
On success: announce via role="status".
On failure: do not clear the form -- preserve user input.

Input Types and Autocomplete

Use semantic type attributes (email, tel, url, number, password). Use autocomplete for user data inputs (WCAG 1.3.5): given-name, family-name, email, tel, street-address, etc.

Disabling Controls

Native disabled attribute removes from tab sequence and announces state.
aria-disabled="true" keeps element focusable -- prevent activation in JS.
Announce dynamic disabled state changes via a live region.

5. Visual Requirements

Text contrast: 4.5:1 minimum (3:1 for large text 18pt+ or 14pt bold+).
UI component contrast: 3:1 minimum for borders, icons, focus indicators.
Never color alone to convey information -- add icons, text, or patterns.
Content must reflow at 320px width without horizontal scroll (WCAG 1.4.10).
No content loss when user adjusts line-height/spacing (WCAG 1.4.12).
Content on hover/focus must be dismissible, hoverable, persistent (1.4.13).
No content flashes more than 3 times/second (2.3.1).
Time limits adjustable; moving content pausable (2.2.1, 2.2.2).

6. Component Patterns

For full ARIA structure, keyboard contracts, and code examples for each pattern, see component-patterns.md. Key rules:

Dialog -- use native <dialog> with .showModal() when possible. Focus trap, Escape to close, return focus to trigger on close.
Tabs -- role="tablist/tab/tabpanel", roving tabindex, arrow keys switch tabs, Tab enters active panel.
Accordion -- trigger is <button> inside a heading, aria-expanded on button, aria-controls to panel.
Disclosure -- <button> with aria-expanded and aria-controls.
Menu button -- aria-haspopup="true", arrows navigate, Enter activates. Never role="menu" for navigation -- use <nav> with links.
Combobox -- aria-activedescendant tracks highlighted option, arrows navigate, Enter selects, Escape closes.
Tooltip -- show on focus + hover, aria-describedby, must be hoverable and persistent. No interactive content inside.
Alert/Status -- role="alert" for urgent (assertive), role="status" for non-urgent (polite). Inject into pre-existing container.

For WCAG 2.2 AA compliance checklist with criterion numbers, see wcag.md.

Application

When writing accessible code:

Apply all conventions silently -- do not narrate each ARIA attribute or WCAG criterion being followed.
Use native HTML elements before reaching for ARIA.
Include keyboard interaction handlers for every custom interactive widget.
If an existing codebase contradicts a convention, follow the codebase pattern and flag the divergence once.

When reviewing code for accessibility:

Cite the specific violation and show the fix inline.
Reference the WCAG criterion number when relevant (e.g., "1.4.3 Contrast").
Do not lecture -- state what is wrong and how to fix it.

Integration

The coding skill governs workflow; this skill governs accessibility choices. For CSS-related accessibility (contrast, focus styles, motion), the css skill complements this one.

accessibility

Accessibility

References