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
;-)