Fix WPT to not assume `button` is the default appearance of a <button>.

There are still a few failures in this test due to us supporting
non-standard aliases of some standard values.

Differential Revision: https://phabricator.services.mozilla.com/D83599

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1652896
gecko-commit: 3a1d1d90b816cc0af52d96c96d0f4ad88aae4a90
gecko-integration-branch: autoland
gecko-reviewers: emilio
diff --git a/css/css-ui/appearance-cssom-001.html b/css/css-ui/appearance-cssom-001.html
index 49769b3..2f06440 100644
--- a/css/css-ui/appearance-cssom-001.html
+++ b/css/css-ui/appearance-cssom-001.html
@@ -24,6 +24,7 @@
                   "textarea",
                   "textfield",
                  ];
+  const initial_appearance = getComputedStyle(button).appearance;
   function assert_style_for_prop(style, prop, value) {
     if (prop === '-webkit-appearance') {
       assert_equals(style.WebkitAppearance, value, 'style.WebkitAppearance (uppercase W)');
@@ -198,7 +199,7 @@
         button.style.setProperty(prop, invalidValue);
         assert_style_for_prop(button.style, prop, "");
         const style = getComputedStyle(button);
-        assert_style_for_prop(style, prop, "button");
+        assert_style_for_prop(style, prop, initial_appearance);
       }, `${prop}: ${invalidValue} (invalid)`);
     }
   }