Moodle uses the TinyMCE 3.x editor which doesn't seem to work with IE11. This problem affects many open source software products and I don't know if it's up to Moodle to fix it.
It probably has to do with the user agent check that in IE11 no longer returns anything with 'MSIE' in.
As a result all users of IE11 see HTML code when they are editing pages, forumposts, etc. (basically everywhere where normally a TinyMCE editor would appear)