That's not gonna happen, the reason they use the video tag is cause it's standard across all browsers and while some don't support a few formats, it's usually cause expensive licensing fees.
Anyway, H.264 MP4 (the same thing YouTube and basically all other sites and cameras use) works in all of them so most people will have 0 issues.
Instead of making a new format, DTube should tell the user their file is unsupported and point the user to a simple tool to convert the video online like ClipChamp.
You opt for a much lesser user friendly solution. I dont mind, I can find my ways, I dont own the DTube service, my suggestion is to make it as user friendly as possible. When DTube (are you part of DTube?) doesn't want to do that, thats up to them. But when DTube wants maximum use of their service? Think as if your grandmother will use the service and know nothing of technology and get super scared when need to start looking for other apps to convert stuff and all. Thats just my 2-cents.
I'm just saying your solution is not possible at all really. No browser is gonna add a special format for one site and even if they do, it would likely take years to convince them and even more years to implement it as a standard in all the browsers. The alternative is having DTube encode the video to a proper format but that requires a central server like YouTube does which is against the whole idea of DTube in the first place.
Meanwhile right now, 99% of videos should work right out of the box and the 1% of people's who don't likely know what they are doing to even have a video file in a different format than the standard to begin with.
I made my comments and suggestions, it is not up to me what will be done, if anything. I can only feedback to the guys at DTube.
For reasons beyond my comprehension you seem to be offended with my suggestions, unvoting a previous comment you upvoted in this chain, as well as your quite hostile comments. I hope you find your ways here at Steemit.