style: Add telemetry for -moz-appearance usage.

Differential Revision: https://phabricator.services.mozilla.com/D77637

components/style/properties/declaration_block.rs

@@ -348,6 +348,21 @@ impl PropertyDeclarationBlock {
             .find(|(declaration, _)| declaration.id() == property)
     }
 
+    /// Get a declaration for a given property with the specified importance.
+    #[inline]
+    pub fn get_at_importance(
+        &self,
+        property: PropertyDeclarationId,
+        importance: Importance,
+    ) -> Option<&PropertyDeclaration> {
+        let (declaration, i) = self.get(property)?;
+        if i == importance {
+            Some(declaration)
+        } else {
+            None
+        }
+    }
+
     /// Tries to serialize a given shorthand from the declarations in this
     /// block.
     pub fn shorthand_to_css(

components/style/properties/longhands/box.mako.rs

@@ -627,6 +627,21 @@ ${helpers.predefined_type(
     gecko_ffi_name="mAppearance",
 )}
 
+// A UA-sheet only property that is always set to the same value as
+// -moz-appearance.  Used to record telemetry for when author sheets
+// override the value of -moz-appearance; see
+// nsIFrame::RecordAppearanceTelemetry.
+${helpers.predefined_type(
+    "-moz-default-appearance",
+    "Appearance",
+    "computed::Appearance::None",
+    engines="gecko",
+    animation_value_type="none",
+    spec="Internal (not web-exposed)",
+    enabled_in="ua",
+    gecko_ffi_name="mDefaultAppearance",
+)}
+
 ${helpers.single_keyword(
     "-moz-orient",
     "inline block horizontal vertical",

components/style/properties/properties.mako.rs

@@ -2111,7 +2111,8 @@ impl PropertyId {
 pub struct WideKeywordDeclaration {
     #[css(skip)]
     id: LonghandId,
-    keyword: CSSWideKeyword,
+    /// The CSS-wide keyword.
+    pub keyword: CSSWideKeyword,
 }
 
 /// An unparsed declaration that contains `var()` functions.