sknebelit's an interesting case, since it evolved from "hacks" to be able to deal with badly written sites, and since the browsers had the code anyways they just allowed it in the spec as well. at least that's my impression ;)
ZegnatHTML did already not comply 100% with SGML (again IIRC), and HTML5 just standardised how to truly parse it (as SGML parsing woulnd’t get you there).