HTML is a fundamental file type.
If XML diffs are implemented per the other UserVoice request, this might not be needed.
Mark Broadhurst commented
I think the problem here is that HTML is a Chomsky Type 2 grammar (context free grammar) and most other programming languages are generally are Chomsky Type 3 grammar (regular expression). Since a Type 2 grammar is fundamentally more complex than a Type 3 grammar (see the Chomsky hierarchy), you would not really get much value out of the types of merge results?
You would be better using an XML based tool like one of Altova's rather something like this.