For video and other synchronized media that includes audio, the captioning requirements depend on what level of conformance you are aiming to reach under the Web Content Accessibility Guidelines (WCAG).
For WCAG 2.1 or 2.2 A conformance, pre-recorded media is usually required to have captions under Success Criterion 1.2.2: Captions (Prerecorded). However, some scenarios are exempt.
If the video’s audio track does not include any meaningful information, captions are not required. Remember that meaningful information isn’t limited to spoken dialog, but can also include sound effects or musical cues. If the audio track does not include meaningful information, it’s best practice to inform users this is the case. An example of an exempt audio track is one that contains only generic background music, and no information would be lost if the track was removed.
If the video is providing a media alternative to text, and is clearly identified as such, then captions are not required. For example, a portion of text is accompanied by a video of a person reading the same text. If this video is clearly labeled as an alternative to the text, then it is not required to have captions. For this exemption to apply, the video must contain no additional information to what is provided in the text.
For WCAG 2.1 or 2.2 AA conformance, live broadcasts of synchronized media (such as video) always require captions under Success Criterion 1.2.4 Captions (Live).
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