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-evaluation.html.ini

@@ -2,9 +2,6 @@
   [Single scope]
     expected: FAIL
 
-  [Scope can not match its own root without :scope]
-    expected: FAIL
-
   [Selecting self with :scope]
     expected: FAIL
 
@@ -26,15 +23,9 @@
   [Multiple scopes from same @scope-rule, only one limited]
     expected: FAIL
 
-  [Multiple scopes from same @scope-rule, both limited]
-    expected: FAIL
-
   [Nested scopes]
     expected: FAIL
 
-  [Nested scopes, reverse]
-    expected: FAIL
-
   [Nested scopes, with to-selector]
     expected: FAIL
 
@@ -47,21 +38,9 @@
   [Simulated inclusive scoping limit]
     expected: FAIL
 
-  [Scope with no elements]
-    expected: FAIL
-
-  [:scope direct adjacent sibling]
-    expected: FAIL
-
-  [:scope indirect adjacent sibling]
-    expected: FAIL
-
   [Relative selector inside @scope]
     expected: FAIL
 
-  [:scope in two different compounds]
-    expected: FAIL
-
   [Scope root with :has()]
     expected: FAIL