style: Add support for is() and where() selectors (#34066)

Additionally, changes now need to be made to `servo.css` in order to
ensure that `<button>` text is centered the same way that it is for
`<input type=button>`. In the past the rule for centering text for the
latter was ignored because it used `:is()`.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

tests/wpt/meta/css/css-cascade/scope-visited-cssom.html.ini

@@ -14,15 +14,9 @@
   [:link as scoping root]
     expected: FAIL
 
-  [:visited as scoping root]
-    expected: FAIL
-
   [:not(:visited) as scoping root]
     expected: FAIL
 
-  [:not(:link) as scoping root]
-    expected: FAIL
-
   [:link as scoping root, :scope]
     expected: FAIL