Safari actually maintains a list of websites where they allow auto-play by default at ~/Library/Safari/SitesAllowedToAutoplay.plist. Most of them are video sites, such as YouTube, Twitch, Netflix, Nicovideo, etc. but they also have some non-video sites that heavily uses video, such as Amazon.

It's a binary plist, but you can view the content using:

    plutil -p ~/Library/Safari/SitesAllowedToAutoplay.plist