Bug 1882581: Allow parsing scope-end selector as relative, anchorin…

…g at `:scope`. r=firefox-style-system-reviewers,emilio

WPT is adjusted to reflect the resolution of Issue #9621
(w3c/csswg-drafts#9621).
That is, relative selectors are serialized with `:scope` e.g.
`> .foo` becomes `:scope > .foo`.

Differential Revision: https://phabricator.services.mozilla.com/D203154

selectors/parser.rs

@@ -438,6 +438,9 @@ pub enum ParseRelative {
     /// Allow selectors to start with a combinator, prepending a parent selector if so. Do nothing
     /// otherwise
     ForNesting,
+    /// Allow selectors to start with a combinator, prepending a scope selector if so. Do nothing
+    /// otherwise
+    ForScope,
     /// Treat as parse error if any selector begins with a combinator.
     No,
 }
@@ -2619,9 +2622,14 @@ where
                 // combinator.
                 builder.push_combinator(combinator.unwrap_or(Combinator::Descendant));
             },
-            ParseRelative::ForNesting => {
+            ParseRelative::ForNesting | ParseRelative::ForScope => {
                 if let Ok(combinator) = combinator {
-                    builder.push_simple_selector(Component::ParentSelector);
+                    let selector = match parse_relative {
+                        ParseRelative::ForHas | ParseRelative::No => unreachable!(),
+                        ParseRelative::ForNesting => Component::ParentSelector,
+                        ParseRelative::ForScope => Component::Scope,
+                    };
+                    builder.push_simple_selector(selector);
                     builder.push_combinator(combinator);
                 }
             },

style/stylesheets/scope_rule.rs

@@ -119,8 +119,7 @@ fn parse_scope<'a>(
                 for_supports_rule: false,
             };
             let parse_relative = if for_end {
-                // TODO(dshin): scope-end can be a relative selector, with the anchor being `:scope`.
-                ParseRelative::No
+                ParseRelative::ForScope
             } else if in_style_rule {
                 ParseRelative::ForNesting
             } else {