Media Accessibility Skill
Reference data for video, audio, and streaming media accessibility. Used by media-accessibility agent and any web agent encountering <video> or <audio> elements.
WCAG 1.2 Time-Based Media Criteria
| SC | Level | Requirement | Test |
|---|---|---|---|
| 1.2.1 | A | Audio-only/video-only: provide text transcript or audio track | Transcript exists alongside media |
| 1.2.2 | A | Captions for prerecorded audio in video | Synchronized captions present |
| 1.2.3 | A | Audio description OR text alternative for prerecorded video | Description track or full transcript |
| 1.2.4 | AA | Captions for live audio in video | Real-time captioning active |
| 1.2.5 | AA | Audio description for prerecorded video | Description track present |
| 1.2.6 | AAA | Sign language for prerecorded audio | Sign language interpretation video |
| 1.2.7 | AAA | Extended audio description | Paused video for long descriptions |
| 1.2.8 | AAA | Media alternative for prerecorded video | Full text alternative document |
| 1.2.9 | AAA | Audio-only (live): text alternative | Real-time text transcript |
Caption File Formats
WebVTT (Web Video Text Tracks)
WEBVTT
00:00:01.000 --> 00:00:04.000
Welcome to the accessibility course.
00:00:04.500 --> 00:00:08.000
Today we'll cover caption best practices.
00:00:08.500 --> 00:00:12.000
<v Speaker 2>Let's start with file formats.Key rules:
- File must start with
WEBVTTheader - Timestamps:
HH:MM:SS.mmm --> HH:MM:SS.mmm - Speaker identification:
<v Speaker Name>Text - Maximum 2 lines per caption, 32 characters per line
- Minimum display time: 1 second
- Maximum display time: 7 seconds
SRT (SubRip Text)
1
00:00:01,000 --> 00:00:04,000
Welcome to the accessibility course.
2
00:00:04,500 --> 00:00:08,000
Today we'll cover caption best practices.Key rules:
- Sequential numbering starting at 1
- Timestamps use comma for milliseconds (not period)
- Blank line between entries
- No styling support (unlike WebVTT)
TTML (Timed Text Markup Language)
- XML-based, used in broadcast and streaming
- Supports styling, positioning, and region layout
- IMSC (Internet Media Subtitles and Captions) is the web profile
Caption Quality Guidelines
| Metric | Requirement |
|---|---|
| Accuracy | 99%+ word accuracy |
| Synchronization | Within 1 second of audio |
| Speaker identification | Required when 2+ speakers |
| Sound effects | Describe in brackets: [applause], [phone rings] |
| Music | Describe: [upbeat music], [dramatic orchestral score] |
| Caption rate | Maximum 200 words per minute (3 words/second) |
| Line length | Maximum 32 characters per line |
| Lines per caption | Maximum 2 lines |
| Placement | Bottom center default; move for on-screen text |
Audio Description
Audio description narrates visual information during pauses in dialogue.
Requirements:
- Describe: actions, scene changes, on-screen text, facial expressions relevant to plot
- Don't describe: obvious audio cues, subjective interpretations
- Timing: fit into natural pauses; for extended descriptions (1.2.7), video pauses automatically
- Voice: distinct from program audio, clear and neutral
HTML implementation:
<video controls>
<source src="video.mp4" type="video/mp4">
<track kind="captions" src="captions-en.vtt" srclang="en" label="English" default>
<track kind="descriptions" src="descriptions-en.vtt" srclang="en" label="English Audio Descriptions">
</video>Accessible Media Player ARIA Patterns
Minimum Controls
Every media player must have keyboard-accessible controls for:
- Play/Pause (
role="button",aria-label="Play"/aria-label="Pause") - Volume (
role="slider",aria-label="Volume",aria-valuemin,aria-valuemax,aria-valuenow) - Seek/Progress (
role="slider",aria-label="Seek",aria-valuetext="2 minutes 30 seconds") - Captions toggle (
role="button",aria-pressed,aria-label="Toggle captions") - Fullscreen (
role="button",aria-label="Enter fullscreen"/aria-label="Exit fullscreen")
Live Region Announcements
<div aria-live="polite" class="sr-only" id="player-status">
<!-- Announce: "Playing", "Paused", "Video ended", "Captions on", "Captions off" -->
</div>Keyboard Shortcuts (Media Player Convention)
| Key | Action |
|---|---|
| Space / Enter | Play/Pause |
| Left Arrow | Rewind 5 seconds |
| Right Arrow | Forward 5 seconds |
| Up Arrow | Volume up |
| Down Arrow | Volume down |
| M | Mute/Unmute |
| C | Toggle captions |
| F | Toggle fullscreen |
Transcript Best Practices
- Provide a full-text transcript adjacent to or linked from the media
- Include speaker identification, timestamps (optional but helpful), and non-speech audio
- Transcripts benefit deaf users, search engines, and users who prefer reading
- Interactive transcripts (click a line to jump to that point) are excellent UX
Live Captioning
- WCAG 1.2.4 (AA) requires captions for live audio in synchronized media
- Options: human captioner (CART), AI-powered auto-captioning, hybrid
- AI auto-captions alone rarely meet the 99% accuracy requirement — human review or correction is recommended
- WebSocket-based caption delivery for web: push text to a
<div aria-live="polite">container
Common Violations
| Issue | WCAG SC | Severity |
|---|---|---|
| No captions on prerecorded video | 1.2.2 | Critical |
| Auto-generated captions without review | 1.2.2 | Serious |
| No audio description for visual-only content | 1.2.5 | Serious |
| Inaccessible media player controls | 2.1.1, 4.1.2 | Critical |
| No keyboard access to play/pause | 2.1.1 | Critical |
| Autoplay without user control | 1.4.2 | Serious |
| Volume slider not keyboard-operable | 2.1.1 | Serious |
| No transcript for audio-only content | 1.2.1 | Critical |
| Captions poorly synchronized (>2s drift) | 1.2.2 | Moderate |
| Missing speaker identification in captions | 1.2.2 | Moderate |