Is that a programming question? Or a general webmaster question? It seems better suited for Doctype.
Jeff Atwood has stated that it is expected that there will be some overlap between this site and StackOverflow and the others. Generally, unless the question is heavily weighted on the code, and less on web development, such questions are fair game on this site.
The question does need to have some relevance to the topic of the site, however. If it's related to the running of a website, then it certainly belongs here.
I think that in general we should apply the following litmus test:
Is this a problem that the typical professional webmaster could be reasonably expected to deal with as part of his work?
In the cited example, I believe this holds true.
This does lead to an overlap with StackOverflow, ServerFault, DocType and perhaps even some other sites but that is simply the nature of the beast.
Re Stack Overflow vs DocType: I have nothing against Doctype but I feel that any HTML/CSS questions fit perfectly fine on Stack Overflow. Without wanting to get into any kind of 'holy war', HTML+CSS = programming. Furthermore SO is more active and users will usually get better and faster answers.
I say: yes, definitely.
My reasoning is this: HTML/CSS from most programmer's perspectives are usually construed as general or even subjective on stackoverflow if they are too basic or actually are subjective. Now unless you throw in the highly-technical details of a
.asp in there, no one is going to want to even give that question a chance.
Now while some questions are duped/overlapped on StackOverflow, DocType or any others... sometimes it is the semantic general question that needs a good answer and for good reason.
I may be a creed apart from the average webmaster, but webmasters.stackexchange is where I go when I am looking for some general feedback, similar experience, or what-did-you-do-when-this-happened...what-did-you-do type of thing.
Quite honestly, I think there is an awesome niche-market for "hey-what-works-best" type of questions, because everyone is new at everything at one time.
I do feel there is going to be some overlap, even some basic-to-technical type of extreme, but this is good, because the general response to me is many many times just as important because I deal with clients that need answers in layman's terms every day.
The other sites that overlap many times are far-far-far too technical to even start to repeat to my clients.
So really, HTML/CSS is a MUST KNOW for webmasters, while language-specific stuff is not, necessarily.
Another option for CSS-specific questions (which the example question certainly is) is the 'CSS expertise' Stack Exchange proposal. It just needs more followers ...
BTW, from the FAQ:
Web design and HTML/CSS layout, and your job title is "designer", ask on Doctype.
so it seems to be (at least, partially) dependant on your job title in addition to the question topic.