Accessibility Engineering for Components

# Accessibility Engineering

Accessibility engineering for product interfaces — from focus states and keyboard support to ARIA, forms, and screen readers. Use when building or reviewing UI components, modals, menus, forms, and custom widgets, or when the user says "make this accessible" or reports keyboard or screen-reader issues.

Accessibility is not a compliance checkbox bolted on at the end; it is the floor for interface craft. Most of it is free if you use the platform: native elements ship with keyboard support, real labels announce themselves, and a visible focus ring is one CSS rule. When reviewing, walk the interface as a keyboard-only user first (every flow must complete without a mouse), then as a screen-reader user — does each control announce a name, a role, and its state? When unsure, prefer the platform default over a custom rebuild, and remove ARIA rather than add it.

## Core Principles

### 1. Native Elements First

The first rule of ARIA: don't use ARIA when a native element exists. `<button>` for actions, `<a href>` for navigation (it must support Cmd/Ctrl/middle-click). Never `<div onClick>`. No ARIA is better than bad ARIA.

### 2. Visible Focus Rings

Style `:focus-visible`, not bare `:focus`, so keyboard users get a ring and mouse users usually don't. Prefer the browser's unmodified focus indicator. If the design needs a custom ring, use a project focus token or an explicit color, and verify the complete indicator against every adjacent color it crosses. Use at least a 2px solid perimeter or an equivalent visible area. Never use `outline: none` without a verified replacement, and preserve system colors in forced-colors mode.

### 3. Full Keyboard Support

Every pointer interaction needs a keyboard path, following the ARIA APG patterns: Escape closes overlays, arrow keys move within composite widgets (tabs, menus, listboxes), Tab moves between widgets, Enter and Space activate. Only `tabindex="0"` (join the natural tab order) and `tabindex="-1"` (programmatic focus) — never positive values, which break the natural order. Composite widgets use roving tabindex: the active item is `0`, all others `-1`.

### 4. Trap and Restore Focus

Modals set `inert` on the background content, move focus inside on open, and return focus to the trigger on close. Add `overscroll-behavior: contain` so background content doesn't scroll.

### 5. Minimum Hit Area

WCAG 2.5.8's Level AA baseline is a 24×24 CSS-pixel target (or a defined exception — spacing, equivalent control, inline, user-agent, or essential). For easier activation, aim for 44×44px in touch contexts and 40×40px on desktop when density permits. Extend with a pseudo-element if the visible element should stay smaller. Never let extended hit areas overlap.

### 6. Label and Type Every Control

Every input gets a `<label for>` or a wrapping `<label>` — a placeholder is never a label, and label and control share one hit target (no dead zone between a checkbox and its text). Add `autocomplete` with a meaningful `name`, and the correct `type`/`inputmode` for the keyboard. Never block paste; users paste passwords and one-time codes.

### 7. Errors That Announce

Keep submit enabled until the request starts, then disable with a spinner while keeping the original label. Validate on submit: mark failing fields with `aria-invalid="true"`, point `aria-describedby` at the inline error text, and focus the first invalid field. Use native `disabled` when a native control is genuinely unavailable; use `aria-disabled="true"` only when retaining focusability or discoverability is intentional, and block pointer/keyboard/form behavior explicitly.

### 8. Accessible Names Everywhere

Icon-only buttons need a descriptive `aria-label`. Visible label text must appear in the accessible name. Decorative elements get `aria-hidden="true"` — never on a focusable element.

### 9. Don't Rely on Color Alone

Status needs a redundant cue: icon, text, or underline alongside the color. Determine which WCAG contrast requirement applies from the content and state, then measure the actual rendered foreground/background pair. When contrast fails, report the pair and requirement it misses — don't change the project's colors unless asked.

### 10. Honor prefers-reduced-motion

Wrap motion in `@media (prefers-reduced-motion: no-preference)` so it's opt-in. Under reduced motion, replace slides and scales with opacity crossfades; kill parallax and autoplay entirely. Independent of the preference: autoplaying media needs a visible pause control, and toasts carrying actions or errors stay until dismissed.

### 11. Announce Dynamic Content

Use `aria-describedby` for field-specific validation, a polite live region (`role="status"`) for non-urgent updates not tied to a control (toasts, result counts), and `role="alert"` only for urgent errors not tied to a control. For reliable repeated polite announcements, render a stable empty region before updating its text — dynamically inserted alerts behave differently across screen readers and must be tested against the target ones.

### 12. Alt Text by Purpose

Decorative images get `alt=""`, informative images describe the meaning, functional images describe the action — a search icon button is `alt="Search"`, not `alt="magnifying glass"`.

### 13. Structure Is Navigation

Use headings that describe their sections and form a coherent outline — one page-level `<h1>` and properly nested levels is the recommended default. Expose one visible primary `<main>` landmark. When repeated navigation or chrome precedes it, make "Skip to content" the first focusable element. Anchored headings get `scroll-margin-top`.

### 14. Survive Zoom and Text Resize

The page must work at 200% zoom and reflow at 320px width without horizontal scrolling. Use `min-height` instead of fixed `height` on text containers, prefer `rem` breakpoints where they fit the codebase's conventions, and never use `user-scalable=no` or `maximum-scale=1`.

## Common Mistakes

| Mistake | Fix |
|---|---|
| `outline: none` to remove the focus ring | Style `:focus-visible` instead; mouse clicks won't show it |
| Custom focus color assumed to work everywhere | Verify the full indicator against every adjacent color and in forced-colors mode |
| `<div onClick>` for a button or link | `<button>` for actions, `<a href>` for navigation |
| Placeholder used as the only label | Add a visible `<label for>`; placeholders disappear on input |
| Positive `tabindex` to fix focus order | Fix the DOM order; only use `0` and `-1` |
| Repeated polite update inconsistently announced | Keep a stable empty status region and update its text; test the target screen readers |
| `assertive` live region for a routine toast | Use `polite`; reserve `assertive` for errors |
| `aria-hidden="true"` on a focusable element | Remove it or make the element non-focusable |
| Functional icon alt describes the picture | Describe the action: `alt="Search"`, not `alt="magnifying glass"` |
| `maximum-scale=1` to stop iOS input zoom | Use a 16px input font on mobile instead; never block zoom |
| Submit disabled until the form is valid | Keep it enabled; validate on submit and focus the first error |

## Review Output Format

Present a standalone review in two parts.

### Findings

Group all confirmed findings by principle. Use a markdown table with **Severity**, **Location**, **Before**, **After**, and **Why** columns — never separate "Before:"/"After:" lines.

- **Severity**: `HIGH` prevents a task, hides content from assistive technology, or is a systemic failure; `MEDIUM` makes an interaction meaningfully harder; `LOW` is isolated polish.
- **Location**: `path/to/file:line`. For an artifact with no source files, cite the exact screen and component instead.
- **Before / After**: the current implementation and an actionable replacement.
- **Why**: name the violated principle and its user impact.

Consolidate a repeated systemic issue into one row listing every affected location. Omit principles with no findings.

**Example:**

| Severity | Location | Before | After | Why |
|---|---|---|---|---|
| HIGH | `src/Dialog.tsx:42` | `<button><XIcon /></button>` | Add `aria-label="Close"`; mark the icon `aria-hidden="true"` | The icon-only control has no accessible name |
| HIGH | `src/button.css:12` | `button:focus { outline: none; }` | `button:focus-visible { outline: 2px solid; outline-offset: 2px; }` | Keyboard users cannot see focus |
| MEDIUM | `src/SignupForm.tsx:64` | Submit disabled until the form is valid | Keep submit enabled; on failure, focus the first invalid field | A disabled action hides what must be fixed |
| MEDIUM | `src/Toolbar.tsx:22` | Icon-only button with a 16px hit area | Extend the hit area to 44×44px with an absolutely-positioned pseudo-element | The target is too small for reliable touch input |

### Verification and Verdict

After the findings: **Verification** — list the exact checks run and their observed results (keyboard traversal, accessible-name inspection, screen-reader or automated checks). State explicitly what still needs verification if a check wasn't run.

**Verdict**: `Block` if any HIGH finding remains, `Needs changes` if only MEDIUM/LOW findings remain, `Approve` only when no actionable findings remain. When there are no findings, omit the tables, state "No actionable accessibility findings," report verification, and end with `Approve`.