Oh they work just fine. Your browser just doesn't support mp4 as noted by my posts above.
I wouldn't say they work just fine. They don't work in 2 browsers with not a huge market share, but still, they don't work. In a lot of cases I'd just say that its the browser/company behind the browser fault, but that would be when they use old tech/standards that slow down progress. Going that far while talking about porn game is a little much but still. In this case, as u pointed out, those browsers don't support mp4 natively because it's not a free standard, and u have to pay someone to use it in your software. And if that standard came with any important benefits I would be all for it. But looking at the sentiment in this forum, with next to no knowledge about the technical side of those formats, it looks like webm offers: similar/same quality, with lower file size and better compatibility.
Dev also said that he watched a tutorial how to resize videos, and resizing is at least a part why the change happened. Recently I tried Shortcut video editor and it seams that changing the new, edited file from mp4 to webm is only choose different file extension from a list. I'd love to stop using Opera and change to some other browser, but most of them have little annoyances, and to those from Opera, I'm at least used to.