[css-properties-values-api] Implement parsing and serialization for @…

…Property at-rule

Implemented behind the new properties-and-values pref.

Support for the CSSPropertyRule WebIDL interface is also added in this
patch, because until it's added, any attempt to access the rule using
the CSSOM would crash the browser.

Depends on D178268

Differential Revision: https://phabricator.services.mozilla.com/D178270

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1833540
gecko-commit: f0ed58537793b76f901d9233f2aaa7f94b5f6e79
gecko-reviewers: emilio

css/css-properties-values-api/at-property.html

@@ -43,15 +43,29 @@
 test_descriptor('syntax', '"<color>"', '<color>');
 test_descriptor('syntax', '"<color> | none"', '<color> | none');
 test_descriptor('syntax', '"<color># | <image> | none"', '<color># | <image> | none');
+test_descriptor('syntax', '"foo | <length>#"', 'foo | <length>#');
 test_descriptor('syntax', '"foo | bar | baz"', 'foo | bar | baz');
-test_descriptor('syntax', '"*"', '*');
 test_descriptor('syntax', '"notasyntax"', 'notasyntax');
 
+// syntax: universal
+for (const syntax of ["*", " * ", "* ", "\t*\t"]) {
+  test_descriptor('syntax', `"${syntax}"`, syntax);
+}
+
 test_descriptor('syntax', 'red', '');
 test_descriptor('syntax', 'rgb(255, 0, 0)', '');
 test_descriptor('syntax', '<color>', '');
 test_descriptor('syntax', 'foo | bar', '');
 
+// syntax: pipe between components
+test_descriptor('syntax', 'foo bar', '');
+test_descriptor('syntax', 'Foo <length>', '');
+test_descriptor('syntax', 'foo, bar', '');
+test_descriptor('syntax', '<length> <percentage>', '');
+
+// syntax: leaading bar
+test_descriptor('syntax', '|<length>', '');
+
 // initial-value
 test_descriptor('initial-value', '10px');
 test_descriptor('initial-value', 'rgb(1, 2, 3)');