In a file foo.html containing

    <!-- foo -->

and in html-mode with point on the "foo", calling `sgml-lexical-context'
returns

    (tag . 1)

where I thought from the docstring of that function it would be

    (comment . 1)

I suppose parse-partial-sexp doesn't quite recognise <!--, or not
without the sgml-specials hack.  I get some joy from the lines below.

2009-10-20  Kevin Ryde  <user42@zip.com.au>

	* textmodes/sgml-mode.el (sgml-lexical-context): Recognise
	comment-start-skip to comment-end-skip as `comment' instead of `tag'.