Our clients gave us a video that is mostly B-roll so they are saying they do not need to add captions to make is ADA compliant, but I'm not sure that is true. I know for images, if it's purely decorative you just leave your alt tag empty, but does the same rule apply to videos? Or do they still need to have captions describing what is going on in the video even though it's mainly decorative?
To comply WCAG 2.0,
Audio description is required only for relevant visuals not already covered in what's said in the video.
It should just describe what is in the visuals so the blind people get a sense of what is going on.
(Audio description says what's in the visuals, so it's available to people who are blind.) Most W3C video won't need audio description. For example, you do not need audio description for talking heads only, or for text on slides as long as the slide text is woven into what you say. You might need audio description of things like charts and diagrams.
Captions - nice to have for most W3C media, required for some. Most W3C media, such as audio-only recordings of teleconferences, will not require captions. Captions are important when people need to see what's happening in the video and get the audio information in text at the same time. Even with videos that are only talking heads, it's good to have captions so that a person who is deaf or hard of hearing can see facial expressions.
Some people will even appreciate captions for audio-only media, for example, if they are hard of hearing or non-native speakers and would like to listen yet also have the text to fill in what they can't hear or understand.
Reference from https://www.w3.org/2008/06/video-notes#q1
W3C WAI recommends that "it's good to let users know" that a video does not need captions or a description, otherwise they may think those were overlooked:
Scroll down to the subhead: "Inform Users When Not Needed"