I'm working on some search engine optimizations on a variety of websites and have noticed that a number of them contain invalid XHTML and are using the XHTML 1.0 Strict DOCTYPE
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
I wonder how much (if at all) this would affect search engines. Any relevant literature would be greatly appreciated.
Matt Cutts from Google said no in 2009. I'd suspect their stance has not changed: