I suppose I'm referring more to 4.01, since 5, as far as I know, doesn't constitute the majority of websites. Things like nonstandard attribute tags and wrapping inline elements around block elements. These are what I thought of when I thought of "dated".
Majority of websites can be converted to 5 just by replacing doctype. Nonstandard attributed don't help you to save bytes though.
Also, from what I've seen sites which were built without even thought about validity and standards tend to be of much lover quality and hence are more bloated: lots of unnecessary div's, classes, wrappers, etc.
