A surprisingly high number of large, well-known websites which are ranked well in Google’s page seem to have lots of W3C validation errors.  Now its common knowledge in the search engine optimisation (SEO) field that older, larger and well known websites have plenty of quality backlinks so W3C web standards compliancy is less important to the SERP listing.

Mozilla Firefox is stricter than Internet Explorer (IE) so shows more errors. Many web developers end up including Cascading Style Sheet (CSS) hacks to improve cross-browser capability and add more functionality, usually to the deteriment of web standards compliancy.