There has been a lot of discussion about what the relationship between TEI 
XML and some forms of HTML might be. 

Such discussion is great, because HTML is likely to remain the dominant 
textual display form for years to come. We need to play nicely with HTML 
(and ePub, which is essentially a carrier for HTML).

However, there are clear semantic differences between HTML and TEI. 

All the semantics additions to  HTML consist of tagging either the entire 
document or some portion of the document text with additional semantics 
over and above the baseline semantics of HTML. This additive model is 
great, because it allows tools to understand HTML without understanding 
all the semantic additions---tools which rely on just the semantics of HTML
can safely ignore all the rest.

Alas, this additive approach assumes only addition. 

When you have tei:ab defined using phrases like "...analogous to, but 
without the semantic baggage of, a paragraph", you have wording that is 
categorically not representable in any of the HTML semantic systems 
that I'm aware of.

cheers
stuart