If I'm reading the Quicktime info correctly the video is using MPEG-4 part 2, the old DivX style codec, rather than the newer MPEG-4 part 10 aka H.264 format which is probably what you want for browser usage. Or maybe using a Youtube embed might be simpler and provide certain other advantages.
That sounds plausible. It has been years since I rendered that video for the web site. I'm sure I did it before H.264 was a thing. I should probably just go back and re-master it. I did put it on YouTube just now but the quality suffered a lot.
Still doesn't make sense to me that it works in some applications and not others.
The download did play in Quicktime, so perhaps the browsers are being slightly more picky about the file format?