That’s why I only validate my template with a strict validation and not the entire website. This way I am sure my template is rock solid before it goes in the CMS or framework. When the production goes CMS, the validation change from strict to transitional because I don’t know who’s working on it next. The client could put his text content inside, and the back-end developer could do minor modifications.
What is alot more important than validation, is for your HTML and css to be well written. In a perfect world where everyone is perfect, strict validation would be perfect, but unfortunatly, there is alot of lazy ass, me first… I think relying only on strict validation doesn’t improve your code, using a good semantic for your html and css however does, anyway there will always be someone to mess up your strict validation.