Extend API to be used in CSS

[romulocintra]

Using the example of User Preference Media Features Client Hints Headers, modelled after CSS' prefers-* that allows :

@media <media-query-list> {
<stylesheet>
}

or language pseudo-class:

:lang( <language-code> ) {}

Would be something to think about having an something that uses locale information or language for condition style

@locale <locale-query-list> {
   // Your style
}