The interrogative pronouns (who/which/what) introduce questions. ( What is that? Who will help me? Which do you prefer?) Which is generally used with more specific reference than what . If we're taking a quiz and I ask " Which questions give you the most trouble?", I am referring to specific questions on that quiz. If I ask " What questions give you most trouble"? I could be asking what kind of questions on that quiz (or what kind of question, generically, in general) gives you trouble. The interrogative pronouns also act as Determiners : It doesn't matter which beer you buy. He doesn't know whose car he hit. In this determiner role, they are sometimes called interrogative adjectives .
As the name suggests, defining relative clauses give essential information to define or identify the person or thing we are talking about. Take for example the sentence: Dogs that like cats are very unusual. In this sentence we understand that there are many dogs in the world, but we are only talking about the ones that like cats. The defining relative clause gives us that information. If the defining relative clause were removed from the sentence, the sentence would still be gramatically correct, but its meaning would have changed significantly.
So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate ) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support . In particular, user agents must not selectively ignore unsupported property values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.