MKV can literally contains anything includes but not limited to text based effected subtitle, image based subtitle, multi alternative audio channel, chapter sep

MKV can literally contains anything includes but not limited to text based effected subtitle, image based subtitle, multi alternative audio channel, chapter separated video and even font. And the used codec also isn't even limited. H265 10bit + flac 96khz? Fine. You can pack it as long as you want. And it's your own problem to play it. And that is basically why you see youtube-dl merges best audio + best video into MKV. Because it is the one only format that allow you to merge something like vp9+opus into playable single file. It is basically a 'one format rule them all' format. The browser is unlikely to actually properly implement all of them. (In fact, there are only a few players actually implement all of them outside of players on windows) And even browser does want to implement everything, there isn't proper API for browser to expose these capabilities to the DOM and JavaScript to properly control them. (I think we are unlikely to see browser support native multi audio track video)