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

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 WEBVTT header
• 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

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)

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