style: Avoid trying to serialize 'all' with stuff that isn't variable references or CSS wide keywords

All is special, and that'd cause an error later on.

Differential Revision: https://phabricator.services.mozilla.com/D115132

components/style/properties/properties.mako.rs

@@ -1537,6 +1537,11 @@ impl ShorthandId {
             return None;
         }
 
+        if self == ShorthandId::All {
+            // 'all' only supports variables and CSS wide keywords.
+            return None;
+        }
+
         // Check whether all declarations can be serialized as part of shorthand.
         if declarations3.all(|d| d.may_serialize_as_part_of_shorthand()) {
             return Some(AppendableValue::DeclarationsForShorthand(self, declarations));