Auto merge of #16205 - bzbarsky:fix-pseudo-preshints, r=Manishearth

Presentation hints should not get applied to pseudo-elements.

Servo side of https://bugzilla.mozilla.org/show_bug.cgi?id=1352464

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components/style/stylist.rs

@@ -622,6 +622,7 @@ impl Stylist {
                                               CascadeLevel::UANormal);
         debug!("UA normal: {:?}", relations);
 
+        if pseudo_element.is_none() {
             // Step 2: Presentational hints.
             let length_before_preshints = applicable_declarations.len();
             element.synthesize_presentational_hints_for_legacy_attributes(applicable_declarations);
@@ -635,6 +636,7 @@ impl Stylist {
                 relations |= AFFECTED_BY_PRESENTATIONAL_HINTS;
             }
             debug!("preshints: {:?}", relations);
+        }
 
         if element.matches_user_and_author_rules() {
             // Step 3: User and author normal rules.