Do not embed auto playing multimedia (either audio or visual).
All multimedia objects require the person to opt in by manually selecting a play button or other engagement element that clearly defines the behavior being initiated. All controls must be properly labeled as well as with ARIA that accurately represents its current state.
Captions are required for all videos with at least 99% accuracy.
Auto captioning services alone do not meet this requirement. Captions support all people, but are specifically necessary for people who are hard of hearing.
Audio description is required where visual content is used to deliver meaning.
To support people who are visually impaired, audio description must accurately represent the content visually presented for the person to have the ability to derive the same meaning from the video content. If the visual content is dense, create a second version of the video with extended audio description that would otherwise be disruptive to other people.
All audio-only content must include transcription.
This supports all people, including those who prefer to read, but especially those who cannot easily hear. With the transcription on a webpage (in HTML), the content also gets an SEO boost, so it can more easily be found through search engines.
Never use video as background
The use of video as background with content overlaid has been proven to be an unfortunate design trend that causes motion sickness and in most cases lack required controls (i.e. play, pause, and stop).