When you have Javascript disabled in Safari, HTML5 video (via the <video> tag) will not play. And the usual “Play”-button often shows a square image instead of a triangle (not always, though). I would have expected for videos to be playable, though, as I have not added any JS myself. (You can still start the video via context menu and “Picture-In-Picture”, though.) You can try on any page that displays videos using a <video> tag. Both on latest Firefox and Chrome, video is playable when Javascript is disabled. Overall, I’m not quite sure whether this is a bug or a feature. But given the square “Play” button and no hint that JS has to be enabled, I think it’s the former in one way or another.
My original description was for the Mac. But I just tested iOS 12 Safari, too, and it’s the same issue there.
*** This bug has been marked as a duplicate of bug 178040 ***