How does Jellyfin-web determine what content a client is capable of playing?

[tmsrxzar]

The error in the log

TranscodeReason=VideoCodecNotSupported, AudioCodecNotSupported

Yet if I set up a simple page testing in the exact same way with canPlayType and test for the codecs that Jellyfin does, I will get no for the 1.1.L120 versions but then I get probably for the 1.1.0.L120 versions, so it really should be direct playing, no?

i have no context to answer this question, you haven't even mentioned your platform, browser or versions
however in my experience browsers make terrible jellyfin clients