I think "Digg's HTML" would actually be wrong. HTML is a language, and the secret isn't hidden at the language but at a markup written in such language. Say the language is called X. Then we'd say: Digg's X markup. Why would it be different with X=HTML?

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.

It might not be an elegant choice of words, but it is not redundant in the same way that "PIN number" is redundant.

It is redundant in precisely the same way.

It is different. "PIN Number" repeats the noun twice. "HTML Markup" uses "markup" twice, but the first instance is a different use than the second instance. In the first instance, it is used as a descriptor for the language. In the second instance, it is used to describe the thing that is written in the language.

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".

In the latter instance, the term you are looking for is not 'Markup' it is 'source' or 'text'.