[hyperhello]

Does this mean I could post some untransliterated text here in a comment and make your browsers all do these computations?

[ars]

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.

[pwdisswordfishq]

Not even any built-in transliteration rules, but the transliteration rules engine. One has to load a custom ruleset to exploit it – and I presume no sane implementation is going to accept rules from untrusted input.

[ks2048]

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

[omoikane]

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#:~:...