The canonical URL of the stylesheet that caused the current canonicalize invocation.
This is only set when the containing stylesheet has a canonical URL, and
when the URL being canonicalized is either relative or has a scheme for
which isNonCanonicalScheme returns
true. This restriction ensures that
canonical URLs are always interpreted the same way regardless of their
Subclasses should only access this from within calls to canonicalize. Outside of that context, its value is undefined and subject to change.
Uri? get containingUrl => utils.containingUrl;