It can, but dealing with raw html, so white / black listing html tags is incredibly difficult and error prone.
I think it'd require whole team that'd maintain that and a lot of tests on different browsers cuz browsers try to "fix" html and it may vary between them, meanwhile it may lead to some bugs(probably)
I'd suggest to try stay away from html as hard as you can and use those cool *down parsers instead :P
So, markdown?
Every website has its own flavor of it. Just reading some of the answers around here you can read "to do markdown better we made our own markdown".
Maybe because it is shitty. Just let users use HTML.