Validation alone is not enough
Some people call me a validation fanatic, and maybe they are right. I do want every HTML document I create and all the sites I build to be valid and remain valid. Validation is an integral part of quality assurance to me, but an HTML document’s validity does not tell the whole story about its quality (there are some examples in Worst code ever).
As Joe Dolson writes in Beyond Validation:
Validation, by itself, should never be considered as a primary signifier concerning the quality or accessibility of a website
Validation is very important. But validation alone is not enough.