I just tried the demo on my very capable laptop (ASUS Zephyrus G15 2021, Linux, Sway; Firefox, also tried Chromium).

Performance is execrable. Text rendering is awful. Input is simply broken (e.g. my Compose key just doesn’t work). Double clicking highlighted the entire canvas thing, as well as the word under the cursor. Right clicking did nothing. Scrolling isn’t captured. The first menu thing I happened to try in the full-UI https://zetaoffice.net/demos/simple-examples/rainbow_writer.... example (Format → Theme, maybe I just picked unfortunately) crashed the whole app (which means that it just stops working mysteriously, leaving the UI in the last state it was in).

And you can’t even get started until it’s downloaded 50MB. (Though I’m actually mildly impressed it’s only that size.)

Seriously, this is completely unusable. It’s “cool tech demo”, but I would hate to actually have to use it.

And I’m pretty sure, based on my rather accurate understanding of how all these things work (comprehensive on the web side, good on the native side, not so much specific about LibreOffice itself), that a lot of this is going to be completely unfixable—though a couple of the things I identified are fairly straightforward to fix, which is if anything a further indictment.

It’s going to be extremely hard to get even decent results without targeting real DOM instead of going pure-canvas, and you can’t get excellent results without doing so.