I am seeing more and more articles written about the importance of HTML and CSS validation. This is in terms of the search engines "liking' your site more if your site validates. So, I did a validation on an out of the box version of product cart BTO prior to any templating. It isn't pretty. Lots of errors. The online demo here did fare a little better, but still shows errors. So, I went next to ("open source, inferior cart") as a comparison: Many of their sites are dead-on "clean".

I know we can clean up our own css files that PC uses now, but some of the items are generated by the store code itself and are "inaccessible" to dummies like me.

Is anyone else putting any work into this? What are the general thoughts on this?

I say it couldn't hurt. 

I decided to try this after reading your post and my company's custom gaming PC sales site, http://www.hipepc.com - http://www.hipepc.com   <--and yes, that was an obvious method of having another link indexed is now listed on the first page of most search engines for search terms related to custom gaming pc sales and sales have gone up something like 300%...Literally o_O

Granted, they've practically chained me to my desk making other optimizations in addition to the 87573489574389 other tasks I take on, but I think it's something of a "sum of its parts" efforts and it wouldn't surprise me if cleaning up the CSS might have helped.

So I thank you greatly for that link and hope you share some more tips with us :)

Woah, THANKS man!!!

I'm in the process of doing a slight redesign, check it out again when you have the chance and THANK YOU :D  I look at the site so much that I can't even tell if it's nice anymore, so it makes me feel really good to hear that!