If you have acquired a legitimate high-quality MKV file, do not try to play it in a browser or an old smart TV app. You need hardware decoding:
This is a direct rip from the streaming source (mostly HBO Max). It is a remux of the original video and audio streams—usually an H.264 or H.265 codec inside an MKV or MP4 container.
When searching for House of the Dragon Season 1 S01 Complete Web-DL High Quality, the acronym "Web-DL" is your most important filter. Here is why:
House of the Dragon — Season 1 Complete (WEB-D | High Quality) — Review, Highlights & Episode Guide