Is validation some kind of quality control? Does "valid" mean "quality approved by W3C"?

Validity is one of the quality criteria for a Web page, but there are many others. In other words, a valid Web page is not necessarily a good web page, but an invalid Web page has little chance of being a good web page.

For that reason, the fact that the W3C Markup Validator says that one page passes validation does not mean that W3C assesses that it is a good page. It only means that a tool (not necessarily without flaws) has found the page to comply with a specific set of rules. No more, no less. This is also why the "valid ..." icons should never be considered as a "W3C seal of quality".

http://validator.w3.org/docs/help.html#validandquality

Ce inseamna un "CSS valid"? Ce versiune de CSS foloseste acest validator?

Conform Specificatia CSS 2.1 : " validitatea unei foi de lucru depinde de nivelul de CSS folosit pentru respectiva foaie de lucru. […] Foaia de lucru CSS 2.1 valida trebuie sa fie scrisa potrivit gramaticii CSS 2.1. Pe langa acestea, trebuie sa contina numai regulile, numele proprietatilor, si valorile proprietatilor definite in aceasta specificatie."

In mod automat, acest validator verifica foile de lucru pentru toate acestea definite in specificatiile CSS 2.1 dar alte profiluri CSS pot fi verificate si dupa alte optiuni

CSS este un limbaj evolutiv, si sunt multi care considera ca "CSS" are o gramatica singulara (cea definita in ultima specificatie) cu un numar de proprietati si valori acceptabile definite in profiluri variate. Versiunile viitoare ale acestui validator ar putea verifica daca foile de lucru corespund ultimelor standarde CSS.
http://jigsaw.w3.org/css-validator/about.html#validCSS