Fullscreen Autoplay Video with sound

Video limitations
There are some key restrictions to this feature, however, especially on mobile devices:

Many browsers, particularly the most recent versions, automatically block videos from automatically playing, particularly ones with audio.
Most mobile devices, including Apple iPhones, Apple iPads and many Android and Microsoft devices do not support the video autoplay feature so your video will not play automatically if a visitor is on one of these devices.
Some desktop or laptop browsers also automatically block videos from automatically playing or may automatically mute the audio.
In some cases, even when the device and browser are compatible with autoplay, the video may not start playing immediately. Typically this is because the rest of the page assets are loading or the video file is still loading or buffering. In most cases, once enough of the page content or video clip has loaded to ensure smoother viewing, the video should begin playing.
In most cases, devices or browsers that do not support autoplay do not, by extension, support looping video.
In Out of the Sandbox themes, there is no way to have a video with audio begin playing with the audio muted. If you do not want to have audio "startle" users, you will need to remove all audio from your video or customize the code.
Also, most mobile devices do not support videos playing behind other elements in Out of the Sandbox themes, so when the user taps the play button, the video will "take over" the entire screen.

The finer points
From an accessibility viewpoint, autoplay can be particularly problematic. Chrome 53 and above on Android provide a setting to disable autoplay completely: from Media settings, select Autoplay.
This change does not affect the audio element: autoplay is still disabled on Chrome on Android, because muted autoplay doesn't make much sense for audio.
There is no autoplay if Data Saver mode is enabled. If Data Saver mode is enabled, autoplay is disabled in Media settings.
Muted autoplay will work for any visible video element in any visible document, iframe or otherwise.
Remember that to take advantage of the new behavior, you'll need to add muted as well as autoplay: compare simpl.info/video with simpl.info/video/muted.