What kind of questions that matters about web(HTML, CSS, PHP, SEO, CMS, ... ) should posted at Stack Overflow and what at Pro Webmasters?
I bumped into this the other day where some folks were trying to close a question about CSS and have it migrated to Pro Webmasters.
The FAQ states:
As a moderator, these are some guidelines I work with based on my interpretation of the FAQs and site-specific meta discourse:
SEO however is on-topic for Pro Webmasters. Pro Webmasters is geared towards running a site as a "Webmaster/Administrator" i.e. site operations but not the minutia of the code or markup itself.
Other CMS systems where you have questions that are programming related should probably be asked on Stack Overflow.
CMS/E-Commerce App questions about the operation, administration and maintenance of your application are a bit of a grey area.
For example, questions about how to set tax rates, managing stock or maintaining content are very application specific and I'd say probably don't have a home anywhere on the Stack Exchange network at this time. There have been a number of Area51 proposals for different CMS applications but none of these on their own have gained enough traction to reach any reasonable critical mass to survive. See: CMS/BLOG proposals MERGER.
On the other hand, questions about sites that happen to have a CMS or E-Commerce application where you're asking an "operational" question about the site overall are probably on-topic.
The bottom line is, check a site's FAQ first. Then if you still can't decide, don't hesitate to ask on a site's meta about whether a question is acceptable.
From what I understand, StackOverflow is for the programming (actual development) for the sites, and ProWebmasters is for when the site is up-and-running. Don't forget there are other semi-related web SE sites, like WordPress Answers.