Playing recorded video before it has been finalized

[ury]

Hi,
Let me start by apologizing in advance if this is duplicated or redundant (as I suspect it should be). I searched and searched, but couldn't find the answer.

I'm looking for MKV a/v container alternative, and "HTML5 video containers" came up. Other than the codec support, the one requirement I have that files can be played while they are being "recorded".
Is this supported by mp4 (I'm almost sure it doesn't), ogg and/or webm?

Thanks in advance,
Ury