Is this SEO company doing things right?

Yes they do seem right, as the goal of 'most' SEO companies is to climb the ranks on a search engine as they promised the client and not to worry about a compliant page for visitors (even though they should).

What I would like to know is what did they do to make a W3C-compliant page go non-compliant

 

I don't worry about compliance at all.

I just check to be sure my site works well in IE and Mozilla. I notice no results in rankings between W3 compliant sites and those that are not.

-Adam

 

I appreciate everyone's responses, now I'm confident about the relation between SEO and W3C.

turfsniffer, it wasn't anything major, simply not using closing tags and outdated tags like <b> and <i>. One thing they did do that I'm very mad about is that they modified a script they shouldn't have even opened, fortunately I caught the mistake and fixed it. So naturally I'm still on edge with this SEO company.

ferret, I've known for a long time about Google's lack of compliance and I know they do it because they need to save on bandwidth from serving billions of pageviews a day.