It's great to have this online, however Wiz made this years ago (except the removing of html , which has been around as $nohtml for a long time) : https://github.com/ds--/CodeArchive/blob/22708365d4c6a6d58b8d2188f22b23c5c0461c60/mSL/net/html2unicode.mrc
Besides, using goto and pipe '|' isn't a good idea (also better to use iswm rather than isin but that's minor), it could make the code significantly slower if used a lot consecutively.
I rewrote wiz's alias to be compatible with mIRC 6.x and to handle unicode in a seperate way : https://dl.dropboxusercontent.com/u/4249275/html2unicode.mrc
It uses an hash table to handle the associated characters to speed up further lookup, use $html2ascii().unicode to enable the unicode support, or just $html2ascii().
It could be a good idea to rewrite it as two version, one for mIRC 7.x and one for mIRC 6.x to speed up the processing even more