The Mastodon video player is bad. Indeed it's one of the worst I've seen, and as the developer of StopTheMadness I've seen a lot of bad video players. You can watch a video posted by me as an example. At this point StopTheMadness can only mitigate the badness of the Mastodon video player. Let me show you the website settings that I use:
I always have "Show video controls" and "Stop autoplaying videos" enabled in the "Default options for all websites". On Mastodon, however, it's necessary to enable "Include loops" too, because for some reason Mastodon adds the loop attribute to all videos.
Another issue is that whenever you hover your mouse over a video, Mastodon attempts to play the video. This happens even when "Autoplay animated GIFs" is disabled in your Mastodon Appearance Preferences. ("Recommended", sigh.) Thus you need to enable "Protect mouse movement". By the way, it's not actually an animated GIF. That's a lie. It's just an MP4 video in a bad video player.
One remaining video issue is that Mastodon somehow cancels "Enter Full Screen". Unfortunately I haven't yet determined the cause. If you want to view a Mastodon video full screen, you're probably better off opening the video in a new tab from the contextual menu.
Before I end this blog post, I have a few non-video notes about Mastodon:
target="_blank" but then clickjacks them to open in your own instance. If StopTheMadness stops the clickjacking, then the links will open in another Mastodon instance instead of your own instance, which is usually not what you want.