Time-based media

Time-based media is content whose understanding depends on time flowing: audio file, video, or both combined. In accessibility, each type of time-based media requires specific alternatives: transcription, captions, audio description. WCAG devotes nine success criteria to it (guideline 1.2).

A podcast without transcription, a training video without captions, a live webinar without real-time captions. Three everyday situations, one common problem.

#What exactly is time-based media?

The term covers all content that unfolds over time. WCAG guideline 1.2 distinguishes four categories:

  • Audio only: podcast, voice message, MP3 file.
  • Video only: silent animation, tutorial without soundtrack.
  • Audio-video: video with a soundtrack (the most common case).
  • Interactive media: audio or video combined with user interaction.

Each category has its own obligations. An audio file alone does not need captions, but it requires a text transcription. A silent video does not need audio description, but it requires an alternative describing what happens on screen.

WCAG adds a second distinction: pre-recorded or live. The nine success criteria (1.2.1 through 1.2.9) specify whether each one applies to live content, recorded content, or both.

#The pitfall most teams ignore

Many teams think "time-based media = video." They add captions and consider the topic settled.

Except a podcast is time-based media. A recorded call is too. Criterion 1.2.1 requires a text transcription for all pre-recorded audio-only content. This is level A, the minimum. Without transcription, a deaf person has no access to the content.

The opposite mistake exists too: providing only captions when information comes through the image. A tutorial that shows on-screen manipulations without describing them orally? Captions are useless for a blind person. You need audio description or, at minimum, a transcription that describes the visual actions.

#Making time-based media accessible in HTML

The <track> tag allows you to associate WebVTT files with a <video> or <audio> element:

<video controls>
  <source src="formation.mp4" type="video/mp4">
  <track kind="captions" src="captions.vtt" srclang="en" label="English" default>
  <track kind="descriptions" src="audiodesc.vtt" srclang="en" label="Audio description">
</video>

The kind attribute makes the difference: captions for captions (dialogue and meaningful sounds), descriptions for audio description.

WCAG devotes significant attention to media. The criteria cover captions, text transcription, synchronized audio description, and absence of auto-playing sound.

#In summary

Time-based media is any content that unfolds over time: audio, video, or both. Each type requires its own alternatives. Captions alone are not enough: think transcription for audio, audio description for visuals.

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Synchronized Audio Description

Synchronized audio description is a voice narration inserted during pauses in a video to describe visual elements that the soundtrack does not convey. It enables people who are blind or visually impaired to follow the action, scene changes, and on-screen text. WCAG requires it at level AA for all prerecorded video content.

Synchronized Audio Description

Synchronized captions are a text version of the dialogue and sounds of a video, displayed in real time in the player. They are not limited to dialogue: they identify who is speaking and describe significant sounds such as music or a door slam. WCAG requires them at Level A for pre-recorded videos (criterion 1.2.2).

Transcript

A transcript is the written version of audio or video content, accessible independently from the player. It reproduces the spoken words, identifies speakers, describes significant sounds, and for videos, the visual information necessary for understanding. WCAG requires it from level A for all prerecorded audio-only content (criterion 1.2.1).