What does "fully valid HTML" get you that "mostly-valid HTML" doesn't?
I'm not trying to be flippant but this just seems like a really silly thing to aim for and an even sillier thing to try to shame people about when ~98% of websites that people use just fine don't do it.
Honestly? I have no idea. But, hey, if it works, it works.
When I met my future wife, the first thing she did was to check if my personal homepage was fully validated. It was. And we are celebrating 20 raucous years this year!