
Matroska isn't supported by the html5 video tag. But I'm just guessing and don't have the knowledge or numbers to back this up.)

(By the way, while I'm not entirely sure, I bet using mpeg2 would possibly be faster to decode than Theora, because FFmpeg likely has more optimizations for such an ubiquitous codec as mpeg. H264 is indeed slower to decode than Theora, but again, you can use Theora in Matroska. You are probably under the mistaken impression that Matroska is inherently slower than OGV, because you've only looked at Matroska files using H264. Matroska isn't slower to read than OGV either. The CPU usage for decoding will be exactly the same. This includes Theora and Vorbis (the codecs you apparently want to use). Matroska itself does support exactly the same codecs as OGV, and more. WebM is different from Matroska (WebM only supports VP8 and VP9.)
