edit: Wouldn't you agree that "Digg's HTML" provides exactly as much information as "Digg's HTML markup"? If so then "markup" is clearly redundant. You're really missing the point by saying that not all markup is HTML. All HTML is markup, it's the HyperText Markup Language. Damn.
edit-ps: It's really annoying that I only see my own message when editing, I should see at least the immediate parent also.
edit2: "HTML" does add information to "markup", but markup doesn't add information to "HTML". "HTML" is a "strict subset" of "markup".
Now the question is, can HTML be used in a sentence about something other than markup, thereby making the use of "markup" in "HTML markup" redundant? I think it can, because one can discuss HTML in general (the rules, the state of the latest version, etc) without referring to some specific markup under discussion.
So, I don't think it is the same kind of thing as "PIN Number" or "HTML Language".
What I aggre though, is that "HTML markup" is redundant in a sense: something written in HTML is a markup, as HTML is a language for markups. So we'd rather say something more general like "HTML code" or "HTML text", as we don't need to specify markup again. But it isn't very important anyway.
edit: http://xkcd.com/386/