[css-cascade] Clarify when an @import rule is valid

[andreubotella]

The definition of when an @import rule is valid is:

Any @import rules must precede all other at-rules and style rules in a style sheet (besides @charset, which must be the first thing in the style sheet if it exists), or else the @import rule is invalid.

Given that in CSS3 @charset is no longer a rule, it's not clear what the interpretation of that sentence would be when the parser finds an invalid rule named @charset that hasn't been interpreted as a fallback encoding marker by the input byte stream.

I've run a few tests, and the behavior of Firefox, Chrome and Epiphany / GNOME Web (a Webkit representative, since I don't have access to Mac or iOS machines) match: an @import rule is valid if every rule before it in the stylesheet is either invalid or an @import rule.

And this behavior is in fact consistent with this text in css-syntax:

If any style rule is invalid, or any at-rule is not recognized or is invalid according to its grammar or context, it’s a parse error. Discard that rule.

But in any case, the language in css-cascade should probably be updated to not refer to @charset as a rule and to clarify this, even if the correct behavior can be reasoned out from other specs.

[fantasai]

That should probably say "all other valid", since invalid stuff should not affect the interpretation of the stylesheet.

[fantasai]

Spec updated and republished.