Hadn’t tried it before but Firefox didn’t want to play it & Chrome thought it should download it.
MP4 seems to work in any browser so if you’re using videos on your own site, that’s the format I’d go with.
Embedded YouTube (and Vimeo, etc) normally work OK as well although some older embed formats (not likely on a new site) can be problematic if someone has disabled Flash for security reasons (highly recommended, it gets rarely gets bug fixes even for quite serious issues.)
That was the experience I had. I got the errors in FireFox on my laptop and then Chrome downloaded it on my desktop machine. I’ll definitely be using MP4s for my own videos. That’s the default format for my Samsung S4. (The quality is surprisingly good. I also have a camcorder but I hafta get a FireWire card for my desktop computer so this is quicker and easier.)
Different browsers handle things a lot differently and that seems to be happening even more with mobile browsing but MP3 for audio and MP4 for video seem to be supported near enough everywhere. So that’s definitely the way to go at the moment.