This comes down to the old question - is 'pure' HTML design (without automatic markup generation or scripting) programming related, or not?
Since HTML is a markup language, not a programming language, many people feel that designing HTML is not programming, and therefore does not belong on SO.
OTOH, I believe that any web developer worth his salt will want to look up the HTML source his code generates from time to time. From this perspective, HTML editors are programming related.
I disagree with Ólafur's answer. Of course, HTML isn't programming. But it is related (rather closely, in fact). Therefore in reply to his statement: Related related related
;-)
Edit In reply to Ólafur's update on his answer:
He is right in saying that there is a gray line, and I agree that a question like How do I install Visual Studio does not belong on SO. Even though installing VS is certainly programming related, such a question is better placed on SU.
The main reason why there still are such questions on SO is that they were asked before SU was available. That can not be used as a justification to post such questions on SO nowadays.