Technically HTML was a derivative of SGML. That said, as someone who did a fair amount of parsing of old-timey HTML...it would have been really nice if it was XML.
Well, by the time the first spec was written it was defined to be an SGML application; when TimBL first implemented it, it was "roughly based on SGML". As far as I'm aware, except for the W3C Validator, no other serious HTML implementation treated HTML as SGML.
And it couldn't have been XML, because HTML predates it.
And it couldn't have been XML, because HTML predates it.