I am unsure if the Forum is familiar with W3C or the World Wide Web Consortium is the main international standards organization for the World Wide Web.
Anyway I have used their free validator services in the past as I found them helpful in correcting little errors (and bigger errors) that I have otherwise not noticed. Even though I am just starting to configure my CE shop, there are many errors according to W3C and I have attached them for comment.
I hope that this information may be useful in the OXID CE development.
Cheers
Don