Since I updated to 10.12.4, whenever I go to a website playing video (tested on YouTube, Vimeo and others), Safari 10.1 takes a few seconds to start the video and then plays it in fullscreen, whether I want it to be fullscreen or not.

This only happens in my main user account, not in my second test user account. I already tested disabling all extensions and launching Safari with shift, but so far, no luck.

Any ideas why that happens? Is there a way to reset Safari or something (without losing my iCloud bookmarks)? I'd like to avoid having to create a new user account and move everything over.