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".