When standards-conscious designers validate their XHTML and CSS templates, everything is nicely compliant up until the point where they start tying in the necessary automated systems like ad software, CMSes, or e-commerce apps. The tools then get in the way and code is needed to fix validation, but a lot of designers dont code.
This is a serious issue if we want to enable standards-based web design or web browsing with wider range of devices like mobile phones and screen readers.
Most Open Source CMSs seem to either make it impossible or difficult to create valid websites with them. For example, when implementing my personal website, I had to make several patches to different MidCOM components to make it validate. And even now some pages have issues, not to mention that the content created with my current WYSIWYG editor doesn't go through any validation or clean-up loop.
This used to be an issue CMS developers didn't think much about. However, as XHTML is becoming more common the level of importance is rising. Dave Shea gives some ideas on how to improve the situation:
What we need is to start working together like this on a larger scale. Youve gone to lengths to programmatically fix improperly nested tags? Great, write it up. You have a killer PHP function for parsing out raw ampersands that can be copied and pasted into a site-wide header? Perfect, share it. You can make a bad tool better? Do it! We dont have to keep re-inventing the wheel for every new site, we can build common code bases that make validation painless and share them.
Valid XHTML output will be important step in the practical CMS interop plan. XHTML and CSS are not yet listed as standards endorsed by OSCOM. While most CMSs deal with more than just the web, these should be the base output standards used for that particular medium.