Fix some subtests in pseudo-classes-after-part.html.

Differential Revision: https://phabricator.services.mozilla.com/D224029

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1921575
gecko-commit: 2fd253587d932b2b8d389b43b73d17374cd8a330
gecko-reviewers: dbaron, firefox-style-system-reviewers, zrhoffman

css/css-shadow-parts/pseudo-classes-after-part.html

@@ -88,7 +88,7 @@
 test_valid_selector("::part(mypart):is(:hover)")
 test_valid_selector("::part(mypart):where(:hover)")
 test_invalid_selector("::part(mypart):not(:first-child)")
-test_valid_selector("::part(mypart):is(:first-child)")
-test_valid_selector("::part(mypart):where(:first-child)")
+test_valid_forgiving_selector("::part(mypart):is(:first-child)")
+test_valid_forgiving_selector("::part(mypart):where(:first-child)")
 
 </script>