Based on
What's the deal with migrating CSS questions to Webmasters?
I updated the Pro Webmasters faq to include
Please note that if your question is about detailed HTML, JavaScript, or CSS coding, it might be a better fit on Stack Overflow. We prefer questions here about problems or issues that affect entire websites.
I don't think it's 100% cut and dried, but it is imperative that users understand Pro Webmasters operates at a higher level, namely, the WHOLE WEBSITE level. This does not rule out HTML / CSS / JavaScript problems, but it certainly raises the bar substantially.
It's probably easier to explain this as "generally disallowed" than it is to get into specifics that might confuse folks, but despite the risk, I think it would be a worthwhile exercise to post a few links to good entire website level HTML / CSS / JavaScript questions here so they can see the difference in scope.