I have a question about some css. Do I post it here? If not here, where?
Try Doctype. That site is targeted at web design questions, which aren't really webmaster-related.
I would actually suggest Stack Overflow. There are currently 23,946 questions tagged
css and 31,479 tagged
html so you'll certainly get good answers more quickly there.
If your question is less code-oriented and more management or "philosophical" related, it is perfectly fine to ask it here on Pro Webmasters.