Replace Namespace components which map to the default namespace with …

…DefaultNamespace.

It seems we don't need to preserve the original prefix name, and this
conveniently leads to_css to now implement CSSOM's requirement to elide
the namespace prefix during serialization when it maps to the default
namespace.

https://drafts.csswg.org/cssom/#serialize-a-simple-selector

components/selectors/parser.rs

@@ -1017,7 +1017,10 @@ fn parse_type_selector<'i, 't, P, E, Impl, S>(parser: &P, input: &mut CssParser<
                     sink.push(Component::DefaultNamespace(url))
                 }
                 QNamePrefix::ExplicitNamespace(prefix, url) => {
-                    sink.push(Component::Namespace(prefix, url))
+                    sink.push(match parser.default_namespace() {
+                        Some(ref default_url) if url == *default_url => Component::DefaultNamespace(url),
+                        _ => Component::Namespace(prefix, url),
+                    })
                 }
                 QNamePrefix::ExplicitNoNamespace => {
                     sink.push(Component::ExplicitNoNamespace)