I’ve spent countless hours at employers fixing Xwiki syntax errors mixed with HTML. The parsing engine must be complex

That's putting it lightly, since Mediawiki templates are Turing-complete.

I'm not up to speed on my parsers anymore, but I believe Parsoid remains the most complete implementation, while mwlib is a reasonable compromise.

https://www.mediawiki.org/wiki/Alternative_parsers#Known_imp...