Draft 3

Add @content to the list of disallowed at-rules.
Allow @media rules within style rules and at-rules. Recent CSS specs allow this, so we shouldn't forbid it.
Disallow silent comments.

Draft 2

Explicitly defined the term "canonical URL" as it relates to stylesheets.
Added a specification for loading plain CSS as an entrypoint file.
Added a non-normative note explaining that the new import algorithm maintains the semantics of the existing import algorithm, other than supporting plain CSS files.