Auto merge of #16895 - bzbarsky:moz-is-html, r=emilio

Add :-moz-is-html support for stylo.

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [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=1365165

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/16895)
<!-- Reviewable:end -->

components/style/gecko/non_ts_pseudo_class_list.rs

@@ -25,8 +25,6 @@
  *
  * Pending pseudo-classes:
  *
- *  :-moz-is-html -> Used only in UA sheets, should be easy to support.
- *
  *  :-moz-lwtheme, :-moz-lwtheme-brighttext, :-moz-lwtheme-darktext,
  *  :-moz-window-inactive.
  *
@@ -108,6 +106,7 @@ macro_rules! apply_non_ts_list {
                 ("-moz-last-node", MozLastNode, lastNode, _, _),
                 ("-moz-only-whitespace", MozOnlyWhitespace, mozOnlyWhitespace, _, _),
                 ("-moz-native-anonymous", MozNativeAnonymous, mozNativeAnonymous, _, PSEUDO_CLASS_INTERNAL),
+                ("-moz-is-html", MozIsHTML, mozIsHTML, _, _),
             ],
             string: [
                 ("-moz-system-metric", MozSystemMetric, mozSystemMetric, _, PSEUDO_CLASS_INTERNAL),

components/style/gecko/selector_parser.rs

@@ -164,7 +164,10 @@ impl NonTSPseudoClass {
         // revalidation, because we already compare states for elements and
         // candidates.
         self.state_flag().is_empty() &&
-        !matches!(*self, NonTSPseudoClass::MozAny(_) | NonTSPseudoClass::Dir(_))
+        !matches!(*self,
+                  NonTSPseudoClass::MozAny(_) |
+                  NonTSPseudoClass::Dir(_) |
+                  NonTSPseudoClass::MozIsHTML)
     }
 
     /// Convert NonTSPseudoClass to Gecko's CSSPseudoClassType.

components/style/gecko/wrapper.rs

@@ -1217,6 +1217,9 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
             NonTSPseudoClass::MozNativeAnonymous => unsafe {
                 Gecko_MatchesElement(pseudo_class.to_gecko_pseudoclasstype().unwrap(), self.0)
             },
+            NonTSPseudoClass::MozIsHTML => {
+                self.is_html_element_in_html_document()
+            }
             NonTSPseudoClass::MozAny(ref sels) => {
                 sels.iter().any(|s| {
                     matches_complex_selector(s, self, context, flags_setter)