Accessibility — Perceivable

WCAG Principle 1: information and user-interface components must be presentable to users in ways they can perceive.

The four sub-criteria, simplified:

Text alternatives for non-text content.
Time-based media (audio/video) has alternatives.
Adaptable content can be presented in different ways without losing structure.
Distinguishable — color contrast, audio control, text resizing.

This skill covers each in practical detail.

1. Text alternatives

Images

Every image gets alternative text describing what's relevant about it. The right alt text depends on the image's role:

<!-- Informative: describe what the user needs to know -->
<img src="/dashboard-screenshot.png"
     alt="Dashboard showing $48,200 monthly revenue, up 12% from last month." />

<!-- Functional (image as link / button): describe the action -->
<a href="/profile">
  <img src="/avatar.png" alt="Open your profile" />
</a>

<!-- Decorative: empty alt so screen readers skip -->
<img src="/decorative-pattern.svg" alt="" aria-hidden="true" />

<!-- Complex (chart, infographic): brief alt + linked long description -->
<figure>
  <img src="/sales-chart.png"
       alt="Quarterly sales chart, see description below."
       aria-describedby="chart-desc" />
  <figcaption id="chart-desc">
    Sales rose from $12k in Q1 to $58k in Q4, with a dip to $31k in Q3 caused
    by a supply chain interruption.
  </figcaption>
</figure>

Common mistakes:

alt="image" or alt="photo of woman" — describes the file type, not the meaning.
Decorative images with non-empty alt — screen reader users hear "decorative-pattern-3-svg" announced as content.
Long alt text on simple images — alt should be ≤ 125 characters; longer goes in <figcaption> or linked description.

Icons

Icon-only buttons need an accessible name. Two patterns:

<!-- Pattern A: aria-label on the button -->
<button aria-label="Close">
  <svg aria-hidden="true">...</svg>
</button>

<!-- Pattern B: visually-hidden text -->
<button>
  <svg aria-hidden="true">...</svg>
  <span class="sr-only">Close</span>
</button>

Either is fine; pattern B can be more reliable across screen readers. The sr-only class:

.sr-only {
  position: absolute;
  width: 1px; height: 1px;
  padding: 0; margin: -1px;
  overflow: hidden;
  clip: rect(0,0,0,0);
  white-space: nowrap;
  border: 0;
}

Decorative SVG

<svg aria-hidden="true" focusable="false">...</svg>

aria-hidden removes from accessibility tree; focusable="false" prevents keyboard focus on the SVG (some browsers focus SVGs by default).

2. Time-based media

Video

Captions for all spoken content. Auto-generated captions are a starting point; review for accuracy.
Audio descriptions for visual content not conveyed by audio (a chart appearing on screen, a character action shown but not narrated).
Transcripts as text alternatives for anyone who can't watch.

Audio

Transcripts for podcasts, interviews, voicemails.
Captions or transcripts for any audio-only message in the UI.

Auto-playing media

Don't autoplay video or audio with sound. WCAG 1.4.2 requires that audio playing for more than 3 seconds either:

Has a pause/stop control, or
Has a volume control independent of system volume.

Better: don't autoplay sound at all. It's hostile to users with hearing aids, in public spaces, or sharing audio output.

3. Adaptable

Semantic structure

Use HTML elements for what they're for. Heading order matters; landmarks matter; lists matter.

<!-- Right -->
<main>
  <h1>Dashboard</h1>
  <section aria-labelledby="kpi-heading">
    <h2 id="kpi-heading">This month</h2>
    <ul>
      <li>Revenue: $48k</li>
      <li>Users: 12,481</li>
    </ul>
  </section>
</main>

<!-- Wrong: divs everywhere, no structure -->
<div>
  <div>Dashboard</div>
  <div>This month</div>
  <div>Revenue: $48k</div>
  <div>Users: 12,481</div>
</div>

Screen readers navigate by headings (H key in NVDA), landmarks (R key), lists, links. Without semantic structure, navigation falls apart.

Heading order

Don't skip heading levels. Page structure:

<h1>Page title (one per page)
  <h2>Section
    <h3>Subsection
      <h4>Sub-subsection

Going from <h1> to <h3> confuses assistive tech. If the visual size you want for an <h2> is too large, restyle the <h2> — don't reach for <h3> to get smaller text.

Reading order

Source order in HTML should match visual reading order. CSS Grid and Flexbox can rearrange visually, but screen readers follow source order.

<!-- Source order matches visual order: title, then summary, then details -->
<article>
  <h2>Article title</h2>
  <p class="summary">Brief summary.</p>
  <p>Detailed body...</p>
</article>

<!-- Wrong: visually rearranged but source order is now confusing -->
<article style="display: flex; flex-direction: column-reverse;">
  <p>Detailed body...</p>
  <p>Summary.</p>
  <h2>Title</h2>
</article>

Programmatic relationships

Use <label for>, aria-labelledby, aria-describedby to expose visual relationships to assistive tech.

<div>
  <label for="email">Email</label>
  <input id="email" aria-describedby="email-help" />
  <p id="email-help">We'll never share this.</p>
</div>

The label is associated; the help text is associated. A screen reader announces all three together.

4. Distinguishable

Color contrast

WCAG ratios:

Content	Level AA	Level AAA
Body text (< 18pt regular or < 14pt bold)	4.5:1	7:1
Large text (≥ 18pt regular or ≥ 14pt bold)	3:1	4.5:1
UI components and graphical objects	3:1	(no AAA)
Disabled / inactive elements	(exempt)	(exempt)

Test every text-on-background combination. Common failures:

Light grey body text (#999 on white = 2.85:1, fails AA).
Brand-color body links (#1e90ff on white = 3.4:1, fails AA for body text).
Placeholder text in inputs (often muted to look "less important," ends up unreadable).

Tools: WebAIM Contrast Checker, Stark plugin, Chrome DevTools' contrast inspector.

Color not the only signal

WCAG 1.4.1: don't use color alone to convey information. Pair with icon, text, pattern, or position.

<!-- Wrong: red border = error, but if red isn't perceivable, no signal -->
<input style="border: 2px solid red" />

<!-- Right: red border + icon + descriptive text -->
<div class="field" data-state="error">
  <label for="email">Email</label>
  <input id="email" aria-invalid="true" aria-describedby="email-error" />
  <p id="email-error">
    <AlertIcon aria-hidden="true" /> Please enter a valid email.
  </p>
</div>

Text resize

WCAG 1.4.4: text must be resizable up to 200% without loss of content or functionality. Test:

Browser zoom at 200% — does content reflow without horizontal scroll?
Text size only at 200% (Firefox supports text-only zoom) — does the layout survive?

Use relative units (rem, em, %) for type sizes; avoid px for body text. Use vw/vh cautiously — they don't scale with text size.

Reflow

WCAG 1.4.10 (Level AA): content reflows to a 320 CSS-pixel-wide viewport without requiring horizontal scrolling. Equivalent to 400% zoom on a 1280px viewport.

Practical test: open the design in DevTools at 320px wide and 1280px wide. Anything that requires sideways scroll for primary content fails.

Images of text

Avoid embedding text in images (which can't be resized, restyled, or read by screen readers). Render text as text wherever possible. SVG with <text> elements is OK because it scales and can be read; raster images of text aren't.

Non-color content

Patterns, textures, position, shape — non-color signals carry meaning for color-blind users:

<!-- Chart series with color + pattern -->
<svg>
  <rect fill="url(#solid-blue)" />     <!-- Series A: solid blue -->
  <rect fill="url(#striped-blue)" />   <!-- Series B: striped blue -->
  <rect fill="url(#dotted-blue)" />    <!-- Series C: dotted blue -->
</svg>

Even users who can perceive color benefit from redundant signals.

Worked example: an accessible status badge system

<style>
  .badge { display: inline-flex; align-items: center; gap: 4px;
           padding: 2px 8px; border-radius: 9999px;
           font-size: 12px; font-weight: 500; }
  .badge--success { background: hsl(142 70% 95%); color: hsl(142 70% 22%); }
  .badge--warning { background: hsl(38 95% 95%); color: hsl(38 90% 25%); }
  .badge--error   { background: hsl(0 80% 95%); color: hsl(0 75% 32%); }
</style>

<span class="badge badge--success">
  <CheckIcon aria-hidden="true" /> Paid
</span>
<span class="badge badge--warning">
  <ClockIcon aria-hidden="true" /> Pending
</span>
<span class="badge badge--error">
  <AlertIcon aria-hidden="true" /> Overdue
</span>

What's working:

Color + icon + text — three signals.
Contrast ratios above 7:1 for both color combinations (verified in a contrast checker).
Icons have aria-hidden since the text already conveys meaning.
Background color tinted, foreground darkened, both relative to a hue rather than using full saturation (preserves AA contrast).

Anti-patterns

alt="image" or alt="" on informative images. Screen reader users get nothing.
Color-only error states. A red border with no text or icon. Color-blind users perceive no error.
Pale-grey body text for elegance. Below 4.5:1 contrast — looks polished, fails AA, exhausts users with low vision.
Auto-playing video with sound. Hostile to users in shared spaces, with hearing aids, on subway commutes.
Heading shenanigans. Using <h2> for "I want this size" rather than for structural meaning. Leaves <h1> missing or <h2> skipped.

Heuristics

The contrast checker pass. Every text-on-background pair, every UI-element-against-background pair. Body must clear 4.5:1; large text and UI 3:1.
The grayscale pass. Take a screenshot, convert to grayscale. Do all status indicators still parse?
The 200% zoom test. Browser zoom to 200%. Layout survives? Buttons still hittable? No content lost?
The screen-reader pass. With VoiceOver / NVDA / TalkBack, walk a critical flow. Listen for unlabeled controls, announced as "blank," missing state.
The image audit. Every <img>. Alt text describes what the user needs to know. Decorative images have empty alt.

Related skills

accessibility (parent).
accessibility-operable — keyboard reach and target size.
accessibility-understandable — labels, copy, predictability.
accessibility-robust — semantic markup and ARIA.
color and hierarchy-color-and-tone (perception) — color decisions that respect contrast.
legibility and readability (perception) — type decisions that aid perception.