WCAG 1.2.4 Captions (Live): How to Provide Real-Time Captions

What This Criterion Requires

WCAG 1.2.4 requires that captions are provided for all live audio content in synchronized media. This applies to live broadcasts, webinars, live-streamed events, virtual meetings, and any other real-time audio presentation that is delivered through a web platform. Unlike prerecorded captions (covered by 1.2.2), live captions must be generated in real time as the audio is being produced. This can be achieved through trained human captioners using CART (Communication Access Realtime Translation) services, automatic speech recognition (ASR) with human correction, or integrated captioning features in streaming platforms. The captions must be synchronized with the audio stream and must include speaker identification, relevant sound effects, and other non-speech audio information that contributes to understanding the content. Organizations that stream live events must plan their captioning workflow in advance to ensure quality and synchronization.

Why It Matters

Live captions are essential for people who are deaf or hard of hearing to participate in real-time events. Without live captions, these users are completely excluded from webinars, live streams, virtual conferences, and live customer support sessions. Unlike prerecorded content that can be captioned after the fact, live events happen in the moment, and missing the information means missing the opportunity to participate, ask questions, and engage. Live captions also benefit users in noisy environments, non-native speakers who process written text more easily than spoken language, and users who prefer to watch content without sound. As remote work and virtual events become the norm, ensuring live captioning is in place is increasingly important for inclusive communication and equal access to information and services.

Common Failures and How to Fix Them

<div class="live-stream">
  <iframe src="https://streaming-service.com/embed/live-event"
    allowfullscreen>
  </iframe>
</div>

<div class="live-stream">
  <iframe src="https://streaming-service.com/embed/live-event?captions=on&captionService=CART"
    allowfullscreen>
  </iframe>
  <p>Live captions are provided by a CART service. 
     If captions are not visible, click the CC button in the player controls.</p>
</div>

<video id="live-player">
  <source src="live-feed.m3u8" type="application/x-mpegURL">
</video>
<script>
  // ASR captions with no quality monitoring
  enableAutoCaption(player, { engine: 'basic-asr' });
</script>

<video id="live-player">
  <source src="live-feed.m3u8" type="application/x-mpegURL">
  <track kind="captions" src="cart-service-endpoint" label="English captions (live)" srclang="en" default>
</video>
<script>
  // CART-assisted captions with human editor for accuracy
  enableLiveCaption(player, {
    service: 'cart-provider',
    humanEditor: true,
    displayStyle: 'overlay-bottom'
  });
</script>

How to Test

1. Join or view the live broadcast and verify that captions appear in real time as speakers talk.
2. Check that captions identify different speakers and include relevant non-speech audio cues such as applause or music.
3. Compare the captions against the actual spoken content to assess accuracy, particularly for technical terms and proper nouns.
4. Verify that caption display timing is closely synchronized with the audio stream, with no more than a few seconds of delay.

CMS-Specific Guidance

This criterion commonly causes issues on these platforms:

• Wordpress Accessibility Checklist
• Shopify Accessibility Checklist
• Squarespace Accessibility Checklist
• Wix Accessibility Checklist
• Webflow Accessibility Checklist

Further Reading

• Video Accessibility Captions Guide
• Wcag Explained Plain English

Related WCAG Criteria

• WCAG 1.2.2
• WCAG 1.2.5