Some might think it's stupid or impossible, but I don't think so. How many simple header/2 columns/footer websites are they? Why are they all differently constructed.. Some uses #contents, others #main-container, others #wrapper, others #wrap, etc etc.
With a great specification for common patterns, it would be possible to document a HTML document by saying, for instance, this page respect the design #18. By that, you could clearly see the doc and it'd show a clear header/3 columns/footer page with all ids and classes used.
It'd make it easier for designers to submit design for project (Think Zen Css for instance). A css could be shared with different websites that use the same 'patterns'.
Think a little bit like the Gammas pattern; some love them, some have them, but we can't deny their usefulness when communicating or documenting a page. For instance, it's way easier to say: I've used the visitor and composite pattern for this tree of blabla rather than describing all the inner details. (Please, don't reply how Gamma patterns are in fact anti-patterns or lack of features of the language, I'm just stating that as example as what I mean by protocol, interface or pattern.)
Finally, it would just plainly make html, css (and maybe even Javascript) code less randomly hacked and easier to maintain. It'll be even easier for beginners to get started. (Take the html code of the pattern #18 and choose a css file from the list associated to the #18) - Dang, a righly constructed 3 columns html page, compliant which work out of the box.