I’ve been to a few websites recently which have autoplaying videos as part of the design. I don’t mean autoplaying video adverts, which are the invention of the devil, but just background video to make the site look cool. If you’re ever tempted by this, be strong, and say no. Website visitors are impatient enough anyway without having to wait for video to load. It is possible to set things up in such a way that the video does not show on small screens, which should relieve people on mobile data from the body blow, but this option often goes unused. Seriously, your site needs to appear quickly, and there’s no way around the fact that video takes time to load.
As for autoplaying video adverts, with sound, I just boycott sites which use these, and so do others. Fortunately from January 2018, with Google’s Chrome browser, autoplay will work only when either there’s no sound in the video or the user has shown interest in the clip. It’s a small but welcome help in the fight against inconsiderate advertisers.