Accessibility means that websites, tools, and technologies are designed and developed so people with disabilities can use them. Once a site has been upgraded and made accessible, it will not play video backgrounds by default. You will need to enable each of them if needed.
To auto-play video background in an accessible site
In the Site Editor, click the video background.
In the left navigation, scroll down until you see the Background section.
Select Video from the tabs (if not automatically selected).
Tick the box to Autoplay video.