Does this mean I could post some untransliterated text here in a comment and make your browsers all do these computations?
I could be wrong, but I don't think it's common for websites to just transliterate any text they're given. Let's check: ウィキペディア
Perhaps I've misunderstood, but I think it's not the text that is unicode complete, it's the transliteration rules that are.
Unless you install special rules you won't be doing extra computation.
I don't think most browsers enable transliteration out of the box.
Lynx does[1], but it does that using iconv, which I think doesn't use UTS #35 transliteration rules.
[1] https://lynx.invisible-island.net/lynx2.9.3/CHANGES.html#:~:...