Hi,

When checking my site at validator.w3.org/nu I get a lot of error messages which looks related to the cleanmart template

Thanks
Michiel

try checking www.google.com or www.facebook.com on w3.org and see how many error you will get.

If google.com or Facebook.com doesn't care about it, why would you?

If your client specifically request this from you, you can clearly explain him/her that w3.org standards are waste of time and doesn't bring any good, beside nobody is caring about it anymore.

Regards.