Just want so say, that got those Katsomo html5 videos work too (without the user agent switcher). I had to enable Media Source API in flags. Also had to enable encrypted media extensions, since some katsomo videos needs those.
I think all of those options are enabled by default in Opera, but i have a (bad) habit of disabling features which i think are not really needed or necessary. Media Source API was causing some high CPU issues, but now it seems to be fine. I'm on latest stable Opera.
Edit: Oh, enabling EME also enabled own plugin for it (widevinecdmadapter.dll).
Widevine Content Decryption Module
Versio: 1.4.8.824
Enables Widevine licenses for playback of HTML audio/video content. (version: 1.4.8.824)