HTML/CSS: practical application is generally considered StackOverflow's domain, theory (as it concerns SEO) is generally considered Webmasters'.
.htaccess/Apache configuration: practical application is generally considered ServerFault's domain, theory (as it concerns SEO) is Webmasters and theory (as it concerns information architecture) should probably belong to UX.
General coding - except as it concerns SEO (Webmasters) or UI (UX) - probably belongs at StackOverflow.
The question you cited was initially submitted at StackOverflow but the asker then became interested in URL rewriting (and copied the bulk of his question over here).
The real confusion lies in the difference between ServerFault (for professional sysadmins - they don't like novice questions) and Webmasters (which is generally more accepting of novice questions), so the example you've provided of a developer looking for advice on IIS administration is actually one of those edge-cases which would be punted to Webmasters from ServerFault if it were posted there and, once at Webmasters, it will certainly not receive the attention that it would at StackOverflow (but its outcomes at StackOverflow will either be an answer ... or a migration to ServerFault, given that many StackOverflow users aren't familiar with ServerFault's attitude toward novice questions).
I don't claim to have a solution for this instance (beyond educating StackOverflow users about Webmasters and trying to soften ServerFault's attitude toward novice questions) but I believe this is an accurate estimate of the situation.