Why is it that when I open up a Silverstripe page in my browser, my selected doctype appears in the source when viewed, however, the W3C validator is picking up a different doctype? Where is this other doctype coming from?
Hmmm... I'm going to hazzard a guess at this one -it's a bit of a leap, stop me if i'm way off ;)
Silverstripe has a Content Negotiator, which, among other things, detects what content you're sending the browser and changes the doctype of the page accordingly. Maybe thats whats got into your pages?
Easy fix and test: add ContentNegotiator::disable(); to your _config.php file to disable the content negotiator and ?flush=1 the site and check if it works now...