Auto merge of #16877 - bzbarsky:moz-anonymous-content, r=emilio

Implement :-moz-anonymous-content in stylo

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix https://bugzilla.mozilla.org/show_bug.cgi?id=1364591

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components/style/matching.rs

@@ -176,7 +176,7 @@ fn element_matches_candidate<E: TElement>(element: &E,
         miss!(State)
     }
 
-    if element.get_id().is_some() {
+    if element.get_id() != candidate_element.get_id() {
         miss!(IdAttr)
     }