The N9 is capable of 720p video playback, right? Fact: the N9 is capable of playing video using its "Wide codec support: H.263, MPEG4-SP & ASP, H.264 BP/MP, WMV9 / VC-1 and Mkv (Matroska)" So the N9 is capable of displaying 720p video, right? Fiction: Not possible. Weasel words from Nokia marketing might make you think that it can, but the N9 display resolution is FWVGA (854 x 480 pixels), but 720p video is 1280x720 pixels. However, the N9 screen is 16:9, so it should show scale wide-screen video quite nicely.
Matroska isn't a video codec. Fact: No, it's not. But let's stay on-topic...
Does the N9 support Flash? This has caused some confusion. The answer is: Fact: the N9 has native support for Flash video. Fiction: the browser does not support Flash (yes yes, just like the iPhone).