style: Avoid selector-matching when only the style attribute is changed.

components/style/restyle_hints.rs

@@ -50,6 +50,14 @@ bitflags! {
     }
 }
 
+impl RestyleHint {
+    /// The subset hints that affect the styling of a single element during the
+    /// traversal.
+    pub fn for_single_element() -> Self {
+        RESTYLE_SELF | RESTYLE_STYLE_ATTRIBUTE
+    }
+}
+
 #[cfg(feature = "gecko")]
 impl From<nsRestyleHint> for RestyleHint {
     fn from(raw: nsRestyleHint) -> Self {