Accessibility Skill | Agent Skills

Accessibility

Overview

Making web UIs usable by everyone, including keyboard-only and assistive-technology users. Covers WCAG 2.1/2.2 AA, semantic HTML, ARIA, keyboard/focus management, screen readers, contrast, and testing. Optimize for AA — that is the legal bar (ADA, EN 301 549, AODA) in most jurisdictions.

Core Rules (internalize these first)

First rule of ARIA: don't use ARIA. A native <button>/<a>/<input>/<nav> ships focus, keyboard, role, and state for free. ARIA only describes; it never adds behavior. Bad ARIA is worse than none.
Second rule: don't change native semantics. <button role="heading"> is a footgun. Don't override roles of interactive elements.
aria-hidden="true" on a focusable element is a trap — it hides the element from the a11y tree while leaving it in the Tab order, so SR users land on "nothing." Hide the whole subtree and remove it from tab order (inert), never one or the other.
Never outline: none without a replacement. Removing the focus ring with no :focus-visible style is the single most common WCAG 2.4.7 failure.
Automated tools catch ~30–40% of issues. axe/Lighthouse find contrast, missing alt/labels, dup IDs — never "is the focus order sane," "does the SR announcement make sense," "is this keyboard-operable." Manual keyboard + SR passes are mandatory.

Accessible Name Computation (the thing people get wrong)

The accessible name is what a screen reader announces. Resolution order (first non-empty wins):

aria-labelledby (space-separated IDs; concatenated text of targets) — highest priority, overrides visible content
aria-label
Native labeling: <label for>/wrapping <label>, <img alt>, <fieldset><legend>, <figcaption>, wrapped text content of a button/link
title / placeholder — fallback only; never rely on these (placeholder vanishes on input, title is not exposed on touch/keyboard reliably)

⚠ Gotchas:

aria-labelledby referencing a hidden (display:none) element still contributes its text — intentional, useful for SR-only names.
aria-label/aria-labelledby are ignored on non-interactive, non-landmark elements (<div>, <span>, <p>) unless they have a role. Don't expect <span aria-label> to announce.
Icon-only controls MUST have a name: <button aria-label="Close">✕</button>. A bare glyph/SVG announces nothing or reads the character.
aria-describedby adds supplementary description (hint, error) read after the name — it does not replace the name.

WCAG: what actually gets flagged (AA)

Grouped by how you catch it. Full spec at w3.org/WAI/WCAG21/quickref; these are the high-frequency failures.

| Area | Criterion | Rule | | ---- | --------- | ---- | | Text alt | 1.1.1 (A) | Every <img> has alt; decorative → alt="" (not missing). Functional image → describe the action. | | Contrast | 1.4.3 (AA) | Body text ≥ 4.5:1; large text (≥18.66px bold / ≥24px) ≥ 3:1. | | Non-text contrast | 1.4.11 (AA) | UI components, focus indicators, and meaningful graphics ≥ 3:1 vs adjacent colors. | | Color alone | 1.4.1 (A) | Never convey info by color only (add icon/text/underline). | | Reflow | 1.4.10 (AA) | Usable at 320px width / 400% zoom, no horizontal scroll. | | Keyboard | 2.1.1 (A) | All functionality via keyboard. | | No trap | 2.1.2 (A) | Focus can leave any component (except intentional modal traps with Esc). | | Focus order | 2.4.3 (A) | Tab order matches visual/reading order. | | Focus visible | 2.4.7 (AA) | Visible focus indicator on every focusable element. | | Name/role/value | 4.1.2 (A) | Custom widgets expose correct role + current state. | | Labels | 3.3.2 (A) | Every input has a programmatic label. | | Error id | 3.3.1 (A) | Errors identified in text (not color) and tied to the field. | | Lang | 3.1.1 (A) | <html lang="…"> set (and lang on inline language switches). |

WCAG 2.2 adds (AA): 2.4.11 Focus Not Obscured (sticky headers must not fully cover the focused element), 2.5.8 Target Size ≥ 24×24px, 3.3.7 Redundant Entry, 3.3.8 Accessible Authentication (no cognitive-test-only auth, allow paste into OTP).

Semantic HTML

Reach for the native element before any <div role>:

<!-- Good: roles, focus, keyboard all free -->
<header>
  <nav aria-label="Main">
    <ul><li><a href="/">Home</a></li><li><a href="/about">About</a></li></ul>
  </nav>
</header>
<main id="main">
  <article><h1>Title</h1><p>…</p></article>
</main>
<footer>…</footer>

One <main> and one <h1> per page; never skip heading levels (h2→h4 breaks the SR outline).
Landmarks (header/nav/main/aside/footer) let SR users jump by region — don't replace them with <div class="header">.
Two same-type landmarks on one page (e.g. two <nav>) MUST be disambiguated with aria-label ("Main"/"Footer").
<button> for actions in-page, <a href> for navigation. A clickable <div> needs role + tabindex="0" + Enter/Space handlers + focus style — i.e. reinventing <button> badly.
Group radios/checkboxes in <fieldset><legend>; <legend> names the group for SR.

ARIA Attributes Reference

// Labels/descriptions
"aria-label" | "aria-labelledby" | "aria-describedby"  // see name computation above
// States (keep in sync with reality on every change)
"aria-expanded": boolean          // disclosure/accordion/combobox trigger
"aria-selected": boolean          // tab/option
"aria-checked": boolean | "mixed" // checkbox/switch/radio
"aria-pressed": boolean | "mixed" // toggle button
"aria-current": "page" | "step" | "location" | "date" | "time" | true // active item in a set
"aria-disabled": boolean          // disabled but still focusable (vs native `disabled`)
"aria-invalid": boolean | "grammar" | "spelling"
"aria-required": boolean
// Live regions
"aria-live": "off" | "polite" | "assertive"
"aria-atomic": boolean            // re-read whole region vs only changed node
// Relationships
"aria-controls" | "aria-owns" | "aria-haspopup": "menu" | "dialog" | "listbox" | true

Landmark roles (prefer the native element that implies them): banner(header), navigation(nav), main, complementary(aside), contentinfo(footer), search, region(needs a name), form.

Live regions — the #1 subtle bug

The container must exist in the DOM before you change its content. Injecting a populated aria-live node in the same tick is often NOT announced — SRs only announce mutations to a region they already observe.

// Wrong: node created+filled together → frequently silent
container.innerHTML = '<div aria-live="polite">Saved</div>';

// Right: region is present and empty on mount; update its text later
<div aria-live="polite" aria-atomic="true" className="sr-only" />  // rendered once
// ...later:  regionEl.textContent = "Saved";

polite = wait for a pause (status, "5 results"). assertive/role="alert" = interrupt immediately (errors only). Overusing assertive is hostile.
role="alert" implies aria-live="assertive" + aria-atomic="true"; role="status" implies polite.
One shared visually-hidden live region toggled via a helper is more reliable than many.

Focus Management

The highest-value, least-automatable a11y work.

Modal open: save document.activeElement, move focus into the dialog (the dialog or its first control), trap Tab inside, close on Esc, restore focus to the trigger on close. Make the background non-interactive with inert (native inert disables focus + pointer + hides from a11y tree in one attribute; falls back to aria-hidden+removing tabbables on older browsers).

SPA route change: browsers reset focus on full loads but NOT on client-side navigation — focus is stranded, and the SR announces nothing. On route change, move focus to the new page's <h1> (give it tabindex="-1") or announce the new title via a live region. This is the most-missed SPA a11y bug.

Roving tabindex (toolbar/menu/tablist/grid): exactly one item has tabindex="0", the rest -1; arrow keys move focus and shift the 0. Keeps composite widgets to a single Tab stop.

// Focus trap core — cycle Tab within a container, Esc to close
function trapFocus(container: HTMLElement, onEscape: () => void) {
  const sel =
    'a[href],button:not([disabled]),input:not([disabled]),select:not([disabled]),textarea:not([disabled]),[tabindex]:not([tabindex="-1"])';
  const onKey = (e: KeyboardEvent) => {
    if (e.key === "Escape") return onEscape();
    if (e.key !== "Tab") return;
    const els = [...container.querySelectorAll<HTMLElement>(sel)];
    const first = els[0], last = els[els.length - 1];
    if (e.shiftKey && document.activeElement === first) { e.preventDefault(); last?.focus(); }
    else if (!e.shiftKey && document.activeElement === last) { e.preventDefault(); first?.focus(); }
  };
  container.addEventListener("keydown", onKey);
  return () => container.removeEventListener("keydown", onKey);
}

⚠ disabled buttons are skipped by Tab and not announced as present — if users must discover a disabled action (e.g. "why can't I submit?"), use aria-disabled="true" + intercept activation instead, so it stays focusable. Prefer native disabled for simple forms.

⚠ Prefer the native <dialog> element (.showModal()): it provides the focus trap, Esc-to-close, backdrop, and top-layer stacking for free — far less to get wrong than a role="dialog" div.

Keyboard Patterns (WAI-ARIA APG)

| Widget | Keys | | ------ | ---- | | Button | Enter and Space activate | | Link | Enter only | | Tabs | ←/→ move between tabs, Home/End first/last, Tab moves to panel (roving tabindex; auto- vs manual-activation) | | Menu / Menubar | ↑/↓ items, →/← submenu, Esc closes + returns focus to trigger, type-ahead | | Combobox | ↓ opens, ↑/↓ options, Enter selects, Esc closes, type filters | | Accordion | Tab between headers, Enter/Space toggles panel | | Dialog | Tab/Shift+Tab trapped, Esc closes, focus restored to trigger | | Radio group | ↑/↓/←/→ move AND select (single Tab stop) |

Implement composite widgets against the WAI-ARIA Authoring Practices patterns rather than inventing key handling. Don't hijack browser/SR shortcuts (single-key handlers can collide with SR quick-nav — gate them behind a modifier or a focused control, per WCAG 2.1.4).

Screen Readers

| Reader | Platform | Notes | | ------ | -------- | ----- | | NVDA | Windows | Free, dominant for testing. H=next heading, K=link, F=form field, D=landmark, NVDA+F7=elements list | | JAWS | Windows | Paid, enterprise. H=heading, Insert+F6=headings list | | VoiceOver | macOS/iOS | Built-in (Cmd+F5). VO=Ctrl+Opt; VO+U=rotor, VO+→/← navigate | | TalkBack | Android | Built-in. Swipe →/← navigate, double-tap activate |

Test matrix that matters: NVDA+Firefox and VoiceOver+Safari cover most real usage. Chrome+NVDA second. Behavior differs across pairs — a name that reads on one may not on another.

SR test checklist:

[ ] Page title announces on load / route change
[ ] Headings form a logical outline (no skipped levels)
[ ] Landmarks present and uniquely labeled; skip link is the first Tab stop
[ ] Images: informative have meaningful alt, decorative have alt=""
[ ] Links/buttons have descriptive names (no bare "click here"/icon-only without label)
[ ] Form fields announce label, required, hint, and error
[ ] Custom widgets announce role + state, and state updates are heard
[ ] Dynamic changes announce via live regions (and only the right ones)

Contrast & Visual

Body ≥ 4.5:1, large text ≥ 3:1, UI/graphics/focus ring ≥ 3:1 (1.4.11).
Contrast ratio = (L_lighter + 0.05) / (L_darker + 0.05), L = relative luminance. Don't hand-roll — use axe, the browser DevTools contrast picker, or a lib; verify against the actual rendered background (gradients/overlays included).
:focus-visible (not :focus) shows the ring for keyboard users without flashing it on mouse click. Never remove the ring without a ≥3:1 replacement.
Respect prefers-reduced-motion — vestibular disorders. Also prefers-contrast, forced-colors (Windows High Contrast: don't set colors via background-image; use currentColor and system color keywords).

:focus-visible { outline: 2px solid #005fcc; outline-offset: 2px; }
@media (prefers-contrast: more) { :focus-visible { outline-width: 3px; } }
@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: .01ms !important; animation-iteration-count: 1 !important;
    transition-duration: .01ms !important; scroll-behavior: auto !important;
  }
}
/* Screen-reader-only text — visible to SR, off-screen visually. NOT display:none (that hides from SR too) */
.sr-only {
  position: absolute; width: 1px; height: 1px; padding: 0; margin: -1px;
  overflow: hidden; clip: rect(0 0 0 0); clip-path: inset(50%); white-space: nowrap; border: 0;
}

Forms

Wire label, hint, error, and validity so a SR reads them as one unit:

<label htmlFor="email">Email <span className="sr-only">(required)</span></label>
<input
  id="email" type="email" autoComplete="email" required
  aria-invalid={!!error}
  aria-describedby={`email-hint${error ? " email-error" : ""}`}
/>
<span id="email-hint" className="hint">We never share your email</span>
{error && <span id="email-error" role="alert">{error}</span>}

Associate every input with a <label for> (or wrap it). Placeholder is not a label.
aria-describedby may list multiple IDs (hint + error) — read in order after the name.
aria-invalid toggles with validity; don't leave it hard-coded true.
On submit failure: move focus to an error summary (tabIndex={-1} + role="alert") with in-page links to each bad field. Don't rely on red borders alone (1.4.1).
Use autocomplete tokens (1.3.5) and correct type/inputmode for mobile keyboards.

Testing

Automated (every PR) — necessary, not sufficient (~30–40% coverage):

// Unit — jest-axe
import { axe, toHaveNoViolations } from "jest-axe";
expect.extend(toHaveNoViolations);
it("no violations", async () => {
  const { container } = render(<MyComponent />);
  expect(await axe(container)).toHaveNoViolations();
});

// E2E — @axe-core/playwright
import AxeBuilder from "@axe-core/playwright";
const results = await new AxeBuilder({ page }).withTags(["wcag2a", "wcag2aa", "wcag21aa"]).analyze();
expect(results.violations).toEqual([]);

Lint: eslint-plugin-jsx-a11y. Query by role/name in tests (getByRole("button", { name: "Save" })) — it fails when the accessible name is missing, doubling as an a11y assertion.
CI gates: Lighthouse categories:accessibility ≥ 0.9, or Pa11y with WCAG2AA.

Manual (before release) — the load-bearing part:

[ ] Unplug the mouse: reach and operate every control by keyboard; focus never lost or trapped
[ ] Focus ring visible on every stop; Tab order matches visual order
[ ] One SR pass (NVDA or VoiceOver): names, roles, states, and live announcements make sense
[ ] Zoom to 400% / 320px width: no loss of content or horizontal scroll
[ ] prefers-reduced-motion and forced-colors modes usable
[ ] Color-only info has a non-color cue

Verify Before Done

[ ] Native element used wherever it fits; ARIA only where HTML can't express it
[ ] Every interactive element has a correct accessible name (icon-only buttons labeled)
[ ] No outline:none without :focus-visible; focus visible everywhere
[ ] Modals: focus trapped, Esc closes, focus restored, background inert/aria-hidden
[ ] SPA route changes move focus and/or announce
[ ] Live regions exist in DOM before updating; polite vs assertive chosen correctly
[ ] No aria-hidden on a focusable element; custom widget states kept in sync
[ ] Form fields wired: label + aria-describedby (hint/error) + aria-invalid
[ ] Contrast: text ≥4.5:1, large/UI ≥3:1; info not conveyed by color alone
[ ] axe/lint green AND one manual keyboard + SR pass done

Quick Reference: ARIA Patterns

<button aria-pressed="true">Mute</button>                      <!-- toggle -->
<button aria-label="Close">✕</button>                          <!-- icon-only -->
<button aria-expanded="false" aria-controls="p1">Toggle</button><div id="p1" hidden>…</div>
<button aria-busy="true">Saving…</button>                       <!-- loading -->
<div role="dialog" aria-modal="true" aria-labelledby="t"><h2 id="t">Title</h2></div>
<div role="alert">Submission failed</div>                       <!-- interrupts -->
<div role="status" aria-live="polite">5 new messages</div>      <!-- polite -->
<a href="#page2" aria-current="page">2</a>                      <!-- active item -->
<span class="sr-only">, opens in new tab</span>                 <!-- SR-only suffix -->
<div aria-hidden="true">★★★</div>                               <!-- decorative, non-focusable -->

Alt text

<img src="chart.png" alt="Sales rose 25% Q1→Q2" />   <!-- informative: describe the info -->
<img src="print.svg" alt="Print this page" />         <!-- functional: describe the action -->
<img src="swoosh.png" alt="" />                        <!-- decorative: empty, not missing -->
<img src="logo.png" alt="Acme Corporation" />          <!-- image of text: transcribe -->
<img src="diagram.png" alt="Architecture overview" aria-describedby="d" /><p id="d">Full description…</p>

Agent Skills: Accessibility

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