I build custom websites that run primarily on css and html. Light JS for applications, and some simple PHP to tie everything together. I am unaware of any flaw or exploit that would disable my sites. I don’t recall any html vulnerabilities. Unless there’s e-commerce involved, that’s nothing I can think of that would be exploitable on either side for html or css. These are both hard coded into the browser so I’m not even sure what an exploit would accomplish other than changing values on the client side. But it’s been a long week…

Edit Looks like zarobi below remembered some exploits. Totally forgot about iframes and the weird image data exploits.