Add 'none' keyword to CSS position-anchor property, and make it the initial value.

Differential Revision: https://phabricator.services.mozilla.com/D272521

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1999972
gecko-commit: 7f71662cdea5d6de048d7e586b6a0b5306cb6670
gecko-reviewers: firefox-style-system-reviewers, layout-reviewers, emilio

Author: jfkthame

css/css-anchor-position/position-anchor-basics.html

@@ -15,23 +15,24 @@
 
 <script>
 // position-anchor: <anchor-element>
-// <anchor-element> = auto | <dashed-ident>
+// <anchor-element> = none | auto | <dashed-ident>
+test_valid_value('position-anchor', 'none');
 test_valid_value('position-anchor', 'auto');
 test_valid_value('position-anchor', '--foo');
-test_invalid_value('position-anchor', 'none');
 test_invalid_value('position-anchor', 'foo-bar');
 test_invalid_value('position-anchor', '--foo --bar')
 test_invalid_value('position-anchor', '--foo, --bar')
 test_invalid_value('position-anchor', '100px');
 test_invalid_value('position-anchor', '100%');
 
 // Computed value: as specified
+test_computed_value('position-anchor', 'none');
 test_computed_value('position-anchor', 'auto');
 test_computed_value('position-anchor', '--foo');
 
-// Initial: auto
+// Initial: none
 // Inherited: no
-assert_not_inherited('position-anchor', 'auto', '--foo');
+assert_not_inherited('position-anchor', 'none', '--foo');
 
 // Animation type: discrete
 test_no_interpolation({