Bug 1886441: Part 3 - Implement substitutions in `@scope`. r=firefox-style-system-reviewers,emilio

`:scope` gets implicitly added if not present, without contributing
specificity (See https://github.com/w3c/csswg-drafts/issues/10196).

`&` is replaced with `scope-start` selector, or `:scope` if it not
specified.

Differential Revision: https://phabricator.services.mozilla.com/D207780

servo/components/malloc_size_of/lib.rs

@@ -791,6 +791,7 @@ where
             Component::Root |
             Component::Empty |
             Component::Scope |
+            Component::ImplicitScope |
             Component::ParentSelector |
             Component::Nth(..) |
             Component::Host(None) |

servo/components/selectors/builder.rs

@@ -97,17 +97,19 @@ impl<Impl: SelectorImpl> SelectorBuilder<Impl> {
         mut spec: SpecificityAndFlags,
         parse_relative: ParseRelative,
     ) -> ThinArc<SpecificityAndFlags, Component<Impl>> {
-        let implicit_parent = parse_relative.needs_implicit_parent_selector() &&
-            !spec.flags.contains(SelectorFlags::HAS_PARENT);
-
-        let parent_selector_and_combinator;
-        let implicit_parent = if implicit_parent {
-            spec.flags.insert(SelectorFlags::HAS_PARENT);
-            parent_selector_and_combinator = [
+        let implicit_addition = match parse_relative {
+            ParseRelative::ForNesting if !spec.flags.intersects(SelectorFlags::HAS_PARENT) => Some((Component::ParentSelector, SelectorFlags::HAS_PARENT)),
+            ParseRelative::ForScope if !spec.flags.intersects(SelectorFlags::HAS_SCOPE | SelectorFlags::HAS_PARENT) => Some((Component::ImplicitScope, SelectorFlags::HAS_SCOPE)),
+            _ => None,
+        };
+        let implicit_selector_and_combinator;
+        let implicit_selector = if let Some((component, flag)) = implicit_addition {
+            spec.flags.insert(flag);
+            implicit_selector_and_combinator = [
                 Component::Combinator(Combinator::Descendant),
-                Component::ParentSelector,
+                component,
             ];
-            &parent_selector_and_combinator[..]
+            &implicit_selector_and_combinator[..]
         } else {
             &[]
         };
@@ -115,11 +117,11 @@ impl<Impl: SelectorImpl> SelectorBuilder<Impl> {
         // As an optimization, for a selector without combinators, we can just keep the order
         // as-is.
         if self.last_compound_start.is_none() {
-            return Arc::from_header_and_iter(spec, ExactChain(self.components.drain(..), implicit_parent.iter().cloned()));
+            return Arc::from_header_and_iter(spec, ExactChain(self.components.drain(..), implicit_selector.iter().cloned()));
         }
 
         self.reverse_last_compound();
-        Arc::from_header_and_iter(spec, ExactChain(self.components.drain(..).rev(), implicit_parent.iter().cloned()))
+        Arc::from_header_and_iter(spec, ExactChain(self.components.drain(..).rev(), implicit_selector.iter().cloned()))
     }
 }
 
@@ -178,6 +180,7 @@ bitflags! {
         const HAS_PARENT = 1 << 3;
         const HAS_NON_FEATURELESS_COMPONENT = 1 << 4;
         const HAS_HOST = 1 << 5;
+        const HAS_SCOPE = 1 << 6;
     }
 }
 
@@ -221,6 +224,18 @@ pub(crate) struct Specificity {
     element_selectors: u32,
 }
 
+impl Specificity {
+    // Return the specficity of a single class-like selector.
+    #[inline]
+    pub fn single_class_like() -> Self {
+        Specificity {
+            id_selectors: 0,
+            class_like_selectors: 1,
+            element_selectors: 0,
+        }
+    }
+}
+
 impl From<u32> for Specificity {
     #[inline]
     fn from(value: u32) -> Specificity {
@@ -311,10 +326,18 @@ where
             Component::Root |
             Component::Empty |
             Component::Scope |
+            Component::ImplicitScope |
             Component::Nth(..) |
             Component::NonTSPseudoClass(..) => {
+                if matches!(*simple_selector, Component::Scope | Component::ImplicitScope) {
+                    flags.insert(SelectorFlags::HAS_SCOPE);
+                }
                 flags.insert(SelectorFlags::HAS_NON_FEATURELESS_COMPONENT);
-                specificity.class_like_selectors += 1;
+                if !matches!(*simple_selector, Component::ImplicitScope) {
+                    // Implicit :scope does not add specificity. See
+                    // https://github.com/w3c/csswg-drafts/issues/10196
+                    specificity.class_like_selectors += 1;
+                }
             },
             Component::NthOf(ref nth_of_data) => {
                 // https://drafts.csswg.org/selectors/#specificity-rules:

servo/components/selectors/matching.rs

@@ -1194,7 +1194,7 @@ where
         Component::Host(ref selector) => {
             return matches_host(element, selector.as_ref(), &mut context.shared, rightmost);
         },
-        Component::ParentSelector | Component::Scope => match context.shared.scope_element {
+        Component::ParentSelector | Component::Scope | Component::ImplicitScope => match context.shared.scope_element {
             Some(ref scope_element) => element.opaque() == *scope_element,
             None => element.is_root(),
         },

servo/components/selectors/parser.rs

@@ -445,19 +445,17 @@ pub enum ParseRelative {
     No,
 }
 
-impl ParseRelative {
-    #[inline]
-    pub(crate) fn needs_implicit_parent_selector(self) -> bool {
-        matches!(self, Self::ForNesting)
-    }
-}
-
 impl<Impl: SelectorImpl> SelectorList<Impl> {
     /// Returns a selector list with a single `&`
     pub fn ampersand() -> Self {
         Self::from_one(Selector::ampersand())
     }
 
+    /// Returns a selector list with a single `:scope`
+    pub fn scope() -> Self {
+        Self::from_one(Selector::scope())
+    }
+
     /// Parse a comma-separated list of Selectors.
     /// <https://drafts.csswg.org/selectors/#grouping>
     ///
@@ -760,6 +758,16 @@ impl<Impl: SelectorImpl> Selector<Impl> {
         ))
     }
 
+    fn scope() -> Self {
+        Self(ThinArc::from_header_and_iter(
+            SpecificityAndFlags {
+                specificity: Specificity::single_class_like().into(),
+                flags: SelectorFlags::HAS_SCOPE,
+            },
+            std::iter::once(Component::Scope),
+        ))
+    }
+
     #[inline]
     pub fn specificity(&self) -> u32 {
         self.0.header.specificity
@@ -780,6 +788,11 @@ impl<Impl: SelectorImpl> Selector<Impl> {
         self.flags().intersects(SelectorFlags::HAS_PARENT)
     }
 
+    #[inline]
+    pub fn has_scope_selector(&self) -> bool {
+        self.flags().intersects(SelectorFlags::HAS_SCOPE)
+    }
+
     #[inline]
     pub fn is_slotted(&self) -> bool {
         self.flags().intersects(SelectorFlags::HAS_SLOTTED)
@@ -1081,6 +1094,7 @@ impl<Impl: SelectorImpl> Selector<Impl> {
                 Root |
                 Empty |
                 Scope |
+                ImplicitScope |
                 Nth(..) |
                 NonTSPseudoClass(..) |
                 PseudoElement(..) |
@@ -1924,6 +1938,14 @@ pub enum Component<Impl: SelectorImpl> {
     Root,
     Empty,
     Scope,
+    /// :scope added implicitly into scoped rules (i.e. In `@scope`) not
+    /// explicitly using `:scope` or `&` selectors.
+    ///
+    /// https://drafts.csswg.org/css-cascade-6/#scoped-rules
+    ///
+    /// Unlike the normal `:scope` selector, this does not add any specificity.
+    /// See https://github.com/w3c/csswg-drafts/issues/10196
+    ImplicitScope,
     ParentSelector,
     Nth(NthSelectorData),
     NthOf(NthOfSelectorData<Impl>),
@@ -2252,13 +2274,14 @@ impl<Impl: SelectorImpl> ToCss for Selector<Impl> {
             debug_assert!(!combinators_exhausted);
 
             // https://drafts.csswg.org/cssom/#serializing-selectors
-            if compound.is_empty() {
-                continue;
-            }
-            if let Component::RelativeSelectorAnchor = compound.first().unwrap() {
+            let first_compound = match compound.first() {
+                None => continue,
+                Some(c) => c,
+            };
+            if matches!(first_compound, Component::RelativeSelectorAnchor | Component::ImplicitScope) {
                 debug_assert!(
                     compound.len() == 1,
-                    "RelativeLeft should only be a simple selector"
+                    "RelativeSelectorAnchor/ImplicitScope should only be a simple selector"
                 );
                 combinators.next().unwrap().to_css_relative(dest)?;
                 continue;
@@ -2530,7 +2553,7 @@ impl<Impl: SelectorImpl> ToCss for Component<Impl> {
             },
             NonTSPseudoClass(ref pseudo) => pseudo.to_css(dest),
             Invalid(ref css) => dest.write_str(css),
-            RelativeSelectorAnchor => Ok(()),
+            RelativeSelectorAnchor | ImplicitScope => Ok(()),
         }
     }
 }
@@ -2613,7 +2636,10 @@ where
                     let selector = match parse_relative {
                         ParseRelative::ForHas | ParseRelative::No => unreachable!(),
                         ParseRelative::ForNesting => Component::ParentSelector,
-                        ParseRelative::ForScope => Component::Scope,
+                        // See https://github.com/w3c/csswg-drafts/issues/10196
+                        // Implicitly added `:scope` does not add specificity
+                        // for non-relative selectors, so do the same.
+                        ParseRelative::ForScope => Component::ImplicitScope,
                     };
                     builder.push_simple_selector(selector);
                     builder.push_combinator(combinator);
@@ -4442,6 +4468,63 @@ pub mod tests {
         assert_eq!(iter.next_sequence(), None);
     }
 
+    #[test]
+    fn test_parse_implicit_scope() {
+        assert_eq!(
+            parse_relative_expected(".foo", ParseRelative::ForScope, None).unwrap(),
+            SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::ImplicitScope,
+                    Component::Combinator(Combinator::Descendant),
+                    Component::Class(DummyAtom::from("foo")),
+                ],
+                specificity(0, 1, 0),
+                SelectorFlags::HAS_SCOPE | SelectorFlags::HAS_NON_FEATURELESS_COMPONENT,
+            )])
+        );
+
+        assert_eq!(
+            parse_relative_expected(":scope .foo", ParseRelative::ForScope, None).unwrap(),
+            SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::Scope,
+                    Component::Combinator(Combinator::Descendant),
+                    Component::Class(DummyAtom::from("foo")),
+                ],
+                specificity(0, 2, 0),
+                SelectorFlags::HAS_SCOPE | SelectorFlags::HAS_NON_FEATURELESS_COMPONENT,
+            )])
+        );
+
+        assert_eq!(
+            parse_relative_expected("> .foo", ParseRelative::ForScope, Some("> .foo")).unwrap(),
+            SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::ImplicitScope,
+                    Component::Combinator(Combinator::Child),
+                    Component::Class(DummyAtom::from("foo")),
+                ],
+                specificity(0, 1, 0),
+                SelectorFlags::HAS_SCOPE | SelectorFlags::HAS_NON_FEATURELESS_COMPONENT,
+            )])
+        );
+
+        assert_eq!(
+            parse_relative_expected(".foo :scope > .bar", ParseRelative::ForScope, None).unwrap(),
+            SelectorList::from_vec(vec![Selector::from_vec(
+                vec![
+                    Component::Class(DummyAtom::from("foo")),
+                    Component::Combinator(Combinator::Descendant),
+                    Component::Scope,
+                    Component::Combinator(Combinator::Child),
+                    Component::Class(DummyAtom::from("bar")),
+                ],
+                specificity(0, 3, 0),
+                SelectorFlags::HAS_SCOPE | SelectorFlags::HAS_NON_FEATURELESS_COMPONENT,
+            )])
+        );
+    }
+
     struct TestVisitor {
         seen: Vec<String>,
     }

servo/components/style/stylist.rs

@@ -39,6 +39,7 @@ use crate::stylesheets::container_rule::ContainerCondition;
 use crate::stylesheets::import_rule::ImportLayer;
 use crate::stylesheets::keyframes_rule::KeyframesAnimation;
 use crate::stylesheets::layer_rule::{LayerName, LayerOrder};
+use crate::stylesheets::scope_rule::ScopeBounds;
 #[cfg(feature = "gecko")]
 use crate::stylesheets::{
     CounterStyleRule, FontFaceRule, FontFeatureValuesRule, FontPaletteValuesRule,
@@ -578,12 +579,13 @@ impl From<StyleRuleInclusion> for RuleInclusion {
 }
 
 /// A struct containing state from ancestor rules like @layer / @import /
-/// @container / nesting.
+/// @container / nesting / @scope.
 struct ContainingRuleState {
     layer_name: LayerName,
     layer_id: LayerId,
     container_condition_id: ContainerConditionId,
     in_starting_style: bool,
+    scope_condition_id: ScopeConditionId,
     ancestor_selector_lists: SmallVec<[SelectorList<SelectorImpl>; 2]>,
 }
 
@@ -595,6 +597,7 @@ impl Default for ContainingRuleState {
             container_condition_id: ContainerConditionId::none(),
             in_starting_style: false,
             ancestor_selector_lists: Default::default(),
+            scope_condition_id: ScopeConditionId::none(),
         }
     }
 }
@@ -605,6 +608,7 @@ struct SavedContainingRuleState {
     layer_id: LayerId,
     container_condition_id: ContainerConditionId,
     in_starting_style: bool,
+    scope_condition_id: ScopeConditionId,
 }
 
 impl ContainingRuleState {
@@ -615,6 +619,7 @@ impl ContainingRuleState {
             layer_id: self.layer_id,
             container_condition_id: self.container_condition_id,
             in_starting_style: self.in_starting_style,
+            scope_condition_id: self.scope_condition_id,
         }
     }
 
@@ -627,6 +632,7 @@ impl ContainingRuleState {
         self.layer_id = saved.layer_id;
         self.container_condition_id = saved.container_condition_id;
         self.in_starting_style = saved.in_starting_style;
+        self.scope_condition_id = saved.scope_condition_id;
     }
 }
 
@@ -2362,6 +2368,23 @@ impl ScopeConditionId {
     }
 }
 
+#[derive(Clone, Debug, MallocSizeOf)]
+struct ScopeConditionReference {
+    parent: ScopeConditionId,
+    // TODO(dshin): With replaced parent selectors, these may be unique...
+    #[ignore_malloc_size_of = "Arc inside"]
+    condition: Option<ScopeBounds>,
+}
+
+impl ScopeConditionReference {
+    const fn none() -> Self {
+        Self {
+            parent: ScopeConditionId::none(),
+            condition: None,
+        }
+    }
+}
+
 /// Data resulting from performing the CSS cascade that is specific to a given
 /// origin.
 ///
@@ -2468,6 +2491,9 @@ pub struct CascadeData {
     /// The list of container conditions, indexed by their id.
     container_conditions: SmallVec<[ContainerConditionReference; 1]>,
 
+    /// The list of scope conditions, indexed by their id.
+    scope_conditions: SmallVec<[ScopeConditionReference; 1]>,
+
     /// Effective media query results cached from the last rebuild.
     effective_media_query_results: EffectiveMediaQueryResults,
 
@@ -2485,6 +2511,22 @@ pub struct CascadeData {
     num_declarations: usize,
 }
 
+fn parent_selector_for_scope(parent: Option<&SelectorList<SelectorImpl>>) -> &SelectorList<SelectorImpl> {
+    lazy_static! {
+        static ref SCOPE: SelectorList<SelectorImpl> = {
+            let list = SelectorList::scope();
+            list.slice()
+                .iter()
+                .for_each(|selector| selector.mark_as_intentionally_leaked());
+            list
+        };
+    };
+    match parent {
+        Some(l) => l,
+        None => &SCOPE,
+    }
+}
+
 impl CascadeData {
     /// Creates an empty `CascadeData`.
     pub fn new() -> Self {
@@ -2510,6 +2552,7 @@ impl CascadeData {
             layer_id: Default::default(),
             layers: smallvec::smallvec![CascadeLayer::root()],
             container_conditions: smallvec::smallvec![ContainerConditionReference::none()],
+            scope_conditions: smallvec::smallvec![ScopeConditionReference::none()],
             extra_data: ExtraStyleData::default(),
             effective_media_query_results: EffectiveMediaQueryResults::new(),
             rules_source_order: 0,
@@ -2867,7 +2910,11 @@ impl CascadeData {
                                 debug_assert!(!has_nested_rules);
                                 debug_assert_eq!(stylesheet.contents().origin, Origin::UserAgent);
                                 debug_assert_eq!(containing_rule_state.layer_id, LayerId::root());
-                                // TODO(dshin, bug 1886441): Because we precompute pseudos, we cannot possibly calculate scope proximity.
+                                // Because we precompute pseudos, we cannot possibly calculate scope proximity.
+                                debug_assert_eq!(
+                                    containing_rule_state.scope_condition_id,
+                                    ScopeConditionId::none()
+                                );
                                 precomputed_pseudo_element_decls
                                     .as_mut()
                                     .expect("Expected precomputed declarations for the UA level")
@@ -2902,7 +2949,7 @@ impl CascadeData {
                             containing_rule_state.layer_id,
                             containing_rule_state.container_condition_id,
                             containing_rule_state.in_starting_style,
-                            ScopeConditionId::none(),
+                            containing_rule_state.scope_condition_id,
                         );
 
                         if collect_replaced_selectors {
@@ -2968,6 +3015,10 @@ impl CascadeData {
                             // NOTE(emilio): It's fine to look at :host and then at
                             // ::slotted(..), since :host::slotted(..) could never
                             // possibly match, as <slot> is not a valid shadow host.
+                            // TODO(dshin, bug 1886441): If we have a featureless
+                            // `:scope`, we need to add to `host_rules` (Which should
+                            // be then rennamed `featureless_rules`).
+                            // See https://github.com/w3c/csswg-drafts/issues/9025
                             let rules = if rule
                                 .selector
                                 .is_featureless_host_selector_or_pseudo_element()
@@ -3194,6 +3245,30 @@ impl CascadeData {
                 CssRule::StartingStyle(..) => {
                     containing_rule_state.in_starting_style = true;
                 },
+                CssRule::Scope(ref rule) => {
+                    let id = ScopeConditionId(self.scope_conditions.len() as u16);
+                    let replaced = {
+                        let start = rule.bounds.start.as_ref().map(|selector| {
+                            match containing_rule_state.ancestor_selector_lists.last() {
+                                Some(s) => selector.replace_parent_selector(s),
+                                None => selector.clone(),
+                            }
+                        });
+                        let end = rule.bounds
+                            .end
+                            .as_ref()
+                            .map(|selector| selector.replace_parent_selector(
+                                parent_selector_for_scope(start.as_ref())));
+                        containing_rule_state.ancestor_selector_lists.push(parent_selector_for_scope(start.as_ref()).clone());
+                        ScopeBounds { start, end }
+                    };
+
+                    self.scope_conditions.push(ScopeConditionReference {
+                        parent: containing_rule_state.scope_condition_id,
+                        condition: Some(replaced),
+                    });
+                    containing_rule_state.scope_condition_id = id;
+                },
                 // We don't care about any other rule.
                 _ => {},
             }
@@ -3386,6 +3461,8 @@ impl CascadeData {
         self.container_conditions.clear();
         self.container_conditions
             .push(ContainerConditionReference::none());
+        self.scope_conditions.clear();
+        self.scope_conditions.push(ScopeConditionReference::none());
         self.extra_data.clear();
         self.rules_source_order = 0;
         self.num_selectors = 0;

testing/web-platform/meta/css/css-cascade/scope-specificity.html.ini

@@ -1,24 +1,33 @@
 [scope-specificity.html]
-  [@scope (#main) { .b {  } }]
+  [@scope (#main) { .b {  } } and .b]
     expected: FAIL
 
-  [@scope (#main) to (.b) { .a {  } }]
+  [@scope (#main) to (.b) { .a {  } } and .a]
     expected: FAIL
 
-  [@scope (#main, .foo, .bar) { #a {  } }]
+  [@scope (#main, .foo, .bar) { #a {  } } and #a]
     expected: FAIL
 
-  [@scope (#main) { div.b {  } }]
+  [@scope (#main) { div.b {  } } and div.b]
     expected: FAIL
 
-  [@scope (#main) { :scope .b {  } }]
+  [@scope (#main) { :scope .b {  } } and .a .b]
     expected: FAIL
 
-  [@scope (#main) { & .b {  } }]
+  [@scope (#main) { & .b {  } } and #main .b]
     expected: FAIL
 
-  [@scope (#main) { div .b {  } }]
+  [@scope (#main) { div .b {  } } and div .b]
     expected: FAIL
 
-  [@scope (#main) { @scope (.a) { .b {  } } }]
+  [@scope (#main) { @scope (.a) { .b {  } } } and .b]
+    expected: FAIL
+
+  [@scope (#main) { :scope .b {  } } and :scope .b]
+    expected: FAIL
+
+  [@scope { & .b {  } } and :scope .b]
+    expected: FAIL
+
+  [@scope (#main) { > .a {  } } and :where(#main) > .a]
     expected: FAIL

testing/web-platform/tests/css/css-cascade/at-scope-parsing.html

@@ -42,9 +42,9 @@
   test_valid('@scope to (.a)');
   test_valid('@scope (.a) to (&)');
   test_valid('@scope (.a) to (& > &)');
-  test_valid('@scope (.a) to (> .b)', '@scope (.a) to (:scope > .b)');
-  test_valid('@scope (.a) to (+ .b)', '@scope (.a) to (:scope + .b)');
-  test_valid('@scope (.a) to (~ .b)', '@scope (.a) to (:scope ~ .b)');
+  test_valid('@scope (.a) to (> .b)');
+  test_valid('@scope (.a) to (+ .b)');
+  test_valid('@scope (.a) to (~ .b)');
   test_valid('@scope ()', '@scope');
   test_valid('@scope to ()', '@scope');
   test_valid('@scope () to ()', '@scope');

testing/web-platform/tests/css/css-cascade/at-scope-relative-syntax.html

@@ -2,6 +2,7 @@
 <title>@scope and Nesting: Parsing inner style rules with relative selector syntax</title>
 <link rel="help" href="https://drafts.csswg.org/css-cascade-6/#scoped-rules">
 <link rel="help" href="https://drafts.csswg.org/css-nesting/#nesting">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/10196">
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <main id=main></main>
@@ -60,9 +61,9 @@
 
   for (const method of Object.keys(create_method)) {
     test_inner(['@scope' , '.nest'], method, '> .foo', '& > .foo');
-    test_inner(['.nest', '@scope'], method, '> .foo', ':scope > .foo');
+    test_inner(['.nest', '@scope'], method, '> .foo');
 
     test_inner(['@scope' , '.nest', '@media screen'], method, '> .foo', '& > .foo');
-    test_inner(['.nest', '@scope', '@media screen'], method, '> .foo', ':scope > .foo');
+    test_inner(['.nest', '@scope', '@media screen'], method, '> .foo');
   }
 </script>

testing/web-platform/tests/css/css-cascade/scope-specificity.html

@@ -1,11 +1,12 @@
 <!DOCTYPE html>
-<title>@scope - specificty</title>
+<title>@scope - specificity</title>
 <link rel="help" href="https://drafts.csswg.org/css-cascade-6/#scope-atrule">
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <style id=style>
 </style>
 <main id=main>
+  <style id=styleImplicit></style>
   <div id=a class=a>
     <div id=b class=b>
     </div>
@@ -36,8 +37,13 @@ function format_scoped_rule(scoped_selector, declarations) {
 // Verify that the specificity of 'scoped_selector' is the same
 // as the specificity of 'ref_selector'. Both selectors must select
 // an element within #main.
-function test_scope_specificity(scoped_selector, ref_selector) {
-  test(() => {
+function test_scope_specificity(scoped_selector, ref_selector, style) {
+  if (style === undefined) {
+    style = document.getElementById("style");
+  }
+  test(t => {
+    t.add_cleanup(() => { style.textContent = ''; });
+
     let element = main.querySelector(ref_selector);
     assert_not_equals(element, null);
 
@@ -62,16 +68,25 @@ function test_scope_specificity(scoped_selector, ref_selector) {
     // cause the unscoped rule to win instead.
     style.textContent = `div${ref_rule} ${scoped_rule}`;
     assert_equals(getComputedStyle(element).zIndex, '2', 'unscoped + scoped');
-  }, format_scoped_rule(scoped_selector, ''));
+  }, format_scoped_rule(scoped_selector, '') + ' and ' + ref_selector);
 }
 
+// Selectors within @scope implicitly have `:scope <descendant-combinator>`
+// added, but no specificity associated with it is added.
+// See https://github.com/w3c/csswg-drafts/issues/10196
 test_scope_specificity(['@scope (#main)', '.b'], '.b');
 test_scope_specificity(['@scope (#main) to (.b)', '.a'], '.a');
 test_scope_specificity(['@scope (#main, .foo, .bar)', '#a'], '#a');
 test_scope_specificity(['@scope (#main)', 'div.b'], 'div.b');
 test_scope_specificity(['@scope (#main)', ':scope .b'], '.a .b');
+// Inherit the specificity of the scope-start selector.
 test_scope_specificity(['@scope (#main)', '& .b'], '#main .b');
 test_scope_specificity(['@scope (#main)', 'div .b'], 'div .b');
 test_scope_specificity(['@scope (#main)', '@scope (.a)', '.b'], '.b');
-
+// Explicit `:scope` adds specficity.
+test_scope_specificity(['@scope (#main)', ':scope .b'], ':scope .b');
+// Using & in scoped style with implicit scope root uses `:scope`, which adds specificity
+test_scope_specificity(['@scope', '& .b'], ':scope .b', styleImplicit);
+// Using relative selector syntax does not add specificity
+test_scope_specificity(['@scope (#main)', '> .a'], ':where(#main) > .a');
 </script>