Very impressive....

I use a number of validators to test my skins, I usually try to achieve at least a 95% compatibility on W3C,HTML5, XHTML5 and etc.

One client where i did this saw their Google Page Rank go up in a span of just a few weeks along with being compatible on most mobile devices.

I have many validators I use to help me narrow down errors in a skin or design and determine if they are show stoppers or if they should be ignored... the goal being at least 95% compatibility although one site I'm working on internally is so far 100% HTML5 / XHTML5 and W3C compliant.... I have not added any 3rd party modules as of yet. That'll knock it down probably...

I ran your impressive skin url (http://design2011.dotnetnuke.com/businessrainbow/Home.aspx) through the following validators:
1) http://www.totalvalidator.com/
2) http://html5.validator.nu/

It was not completely error free but I saw no real show stoppers or nothing that couldn't be fixed.

Again nice work. What Validators do you use to check your skins?

But my biggest concern is the number of skins or modules on the DNN Store that claim they are HTML5/XHTML5 and W3C compliant charge HIGH costs and when you get the skin, you find that its actually only 60%-70% compliant or compatible and you're stuck trying to tweak them into some level of web safe compliant compatibility.

Yes, that is my approach as well, I validate the skin first then retro fit over to DNN. Depending on how the module validates, I either fix the css or if the Module fails majorly, I just drop that module completely.

Along with that,I've taught my clients to keep their CMS updates simple along with re-running the validation.
That way if I ever get a complaint that a DNN web site is out of compliance, I can cross check with a "before" and "after" in order to educate them on keeping their CMS entries clean if at all possible....So far this approach has worked for me.