I’ve tried two different setups for Hangul input.

The first combination (GNOME+X11+IBus) just worked—it was trivial to setup from the GNOME Control Center and there were no issues that I could find. Last time I tried this was around a year ago but it had been working for years before that.

The second combination (Plasma+Wayland+IBus) barely worked: I could enter characters but couldn’t add space or other symbols between characters, and settings were all over the place and would randomly stop working. Last tested ten minutes ago.

I’ve heard people getting better results on Plasma+Wayland+Fcitx5 but it’s not something I’ve tried.

I don’t think the reason was technical. Firefox has supported WebM (a subset of Matroska) for 11 years, and whatever code they had probably would have been enough for most Matroska files, assuming the codecs are also supported.

However, Matroska itself was only officially standardised last October despite being in use all these years. That was probably what convinced them to add support.