Yes, they know.. But I think, doesn't matter about this.. I think, you must concern to build your niche site, and make seo.
If you mean 100% comply with W3C standard for maximum SEO output then I would recommend you should not pay too much attention on it. I do agree that badly on W3C standard will make your website appearance weird and search engine bots may not crawl properly. However, you could find yourself in trouble for such a coding for rarely time. Most of websites has a minor error in W3C perspecive, but search engine bots could still crawl on most pages without a problem. Bottom Line : Your website can still have a good ranking without 100% comply with W3C. Even Google is still not 100% comply with W3C standard. lol
Just like any site extension that ranks well, page extension doesn't matter too when it comes to eye of bots, they just love fresh contents, quality contents and good linking structure of sites irregardless how they are made...