Auto merge of #25627 - pshaughn:definedpseudo, r=emilio,jdm

Implement :defined CSS selector

<!-- Please describe your changes on the following line: -->
ElementState::IN_DEFINED_STATE already existed but wasn't hooked up to anything; now it is set appropriately, and CSS is aware of it. The main test for this selector needed the window named getter for no important reason, so I made a one-line test change to avoid that. Remaining failures in the test are all about SVG elements.

---
<!-- 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 #25010

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

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2 files changed, 14 insertions, 2 deletions

diff --git a/components/script/dom/create.rs b/components/script/dom/create.rs
index d67550ee47f..f68a680483d 100644
--- a/components/script/dom/create.rs
+++ b/components/script/dom/create.rs
@@ -200,6 +200,8 @@ fn create_html_element(
         None => {
             if is_valid_custom_element_name(&*name.local) {
                 result.set_custom_element_state(CustomElementState::Undefined);
+            } else {
+                result.set_custom_element_state(CustomElementState::Uncustomized);
             }
         },
     };
diff --git a/components/script/dom/element.rs b/components/script/dom/element.rs
index 29bdaf2f3bf..c26e492d544 100644
--- a/components/script/dom/element.rs
+++ b/components/script/dom/element.rs
@@ -324,14 +324,23 @@ impl Element {
     }
 
     pub fn set_custom_element_state(&self, state: CustomElementState) {
-        self.ensure_rare_data().custom_element_state = state;
+        // no need to inflate rare data for uncustomized
+        if state != CustomElementState::Uncustomized || self.rare_data().is_some() {
+            self.ensure_rare_data().custom_element_state = state;
+        }
+        // https://dom.spec.whatwg.org/#concept-element-defined
+        let in_defined_state = match state {
+            CustomElementState::Uncustomized | CustomElementState::Custom => true,
+            _ => false,
+        };
+        self.set_state(ElementState::IN_DEFINED_STATE, in_defined_state)
     }
 
     pub fn get_custom_element_state(&self) -> CustomElementState {
         if let Some(rare_data) = self.rare_data().as_ref() {
             return rare_data.custom_element_state;
         }
-        CustomElementState::Undefined
+        CustomElementState::Uncustomized
     }
 
     pub fn set_custom_element_definition(&self, definition: Rc<CustomElementDefinition>) {
@@ -3039,6 +3048,7 @@ impl<'a> SelectorsElement for DomRoot<Element> {
             NonTSPseudoClass::Focus |
             NonTSPseudoClass::Fullscreen |
             NonTSPseudoClass::Hover |
+            NonTSPseudoClass::Defined |
             NonTSPseudoClass::Enabled |
             NonTSPseudoClass::Disabled |
             NonTSPseudoClass::Checked |