I’ve tested a few, the ones I’ve found that work are;
<color=#000000> text changes colour to the hex value
<mark> text <mark/> highlights text like this but in yellow
<sub> text </sub> makes something a subtext like this
<sup> text </sub> makes something a super text like this
<u> text <u> underlines text like this
<b> text </b> makes text bold like this (this one is hard to notice in game)
<i> text <i> makes text italic like this
these are all html tags there’s plenty of places to find more but I’ve tried most of them and a lot wouldn’t make sense/likely wont work at all on a vrchat avatar.
you can also use multiple colours in a single line of text like this by putting a colour tag before each letter.