Funny enough, the one thing XMPP does not use xml for is markup! See
https://xmpp.org/extensions/xep-0393.html - an ad hoc markup format
https://xmpp.org/extensions/xep-0394.html - a laughably perverse usage of the already perverse XML format I don't know why they did this but I guess they had reasons - perhaps XML doesn't actually do that well even for markups
Once upon a time, we had XHTML-IM https://xmpp.org/extensions/xep-0071.html
But then web developers came along and just put this directly into the DOM of their web clients, leading to endless XXS exploits, so XEP-0071 was burned at the stake.
XEP-0393 might look ad-hoc, but it's essentially what people were typing into their chats and emails since time immemorial.
People sometimes think this is Markdown and then pick a markdown library off the shelf and then the HTML passtrough bites them, leading us back to the beginning.
I really don't understand how Matrix and Mastodon etc are allowed to pass around HTML embedded in JSON as if that somehow solves all those problems.
Maybe one day xhtml-im will make a glorious return as a 2.0, with bigger, better and scarier warnings about sanitizing your inputs