Auto merge of #20010 - emilio:pres-hints-cascade, r=bholley

style: Cascade pres hints after normal user rules.

Per https://drafts.csswg.org/css-cascade/#preshint and
https://html.spec.whatwg.org/#presentational-hints.

This was causing failures in the link color reftests with the preferences sheet
as a User sheet.

Bug: 1436782
Reviewed-by: bholley
MozReview-Commit-ID: 9iwEqPBw4CF

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2 files changed, 43 insertions, 33 deletions

diff --git a/components/style/rule_tree/mod.rs b/components/style/rule_tree/mod.rs
index 9c710f07c5f..0ff2dbf28dc 100644
--- a/components/style/rule_tree/mod.rs
+++ b/components/style/rule_tree/mod.rs
@@ -472,22 +472,28 @@ impl RuleTree {
 /// where it likely did not result from a rigorous performance analysis.)
 const RULE_TREE_GC_INTERVAL: usize = 300;
 
-/// The cascade level these rules are relevant at, as per[1].
+/// The cascade level these rules are relevant at, as per[1][2][3].
+///
+/// Presentational hints for SVG and HTML are in the "author-level
+/// zero-specificity" level, that is, right after user rules, and before author
+/// rules.
 ///
 /// The order of variants declared here is significant, and must be in
 /// _ascending_ order of precedence.
 ///
 /// [1]: https://drafts.csswg.org/css-cascade/#cascade-origin
+/// [2]: https://drafts.csswg.org/css-cascade/#preshint
+/// [3]: https://html.spec.whatwg.org/multipage/#presentational-hints
 #[repr(u8)]
 #[derive(Clone, Copy, Debug, Eq, PartialEq, PartialOrd)]
 #[cfg_attr(feature = "servo", derive(MallocSizeOf))]
 pub enum CascadeLevel {
     /// Normal User-Agent rules.
     UANormal = 0,
-    /// Presentational hints.
-    PresHints,
     /// User normal rules.
     UserNormal,
+    /// Presentational hints.
+    PresHints,
     /// Author normal rules.
     AuthorNormal,
     /// Style attribute normal rules.
diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index 2018468207f..18e5fb4c7d1 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -1157,7 +1157,10 @@ impl Stylist {
 
     /// Returns the applicable CSS declarations for the given element.
     ///
-    /// This corresponds to `ElementRuleCollector` in WebKit.
+    /// This corresponds to `ElementRuleCollector` in WebKit, and should push to
+    /// elements in the list in the order defined by:
+    ///
+    /// https://drafts.csswg.org/css-cascade/#cascade-origin
     pub fn push_applicable_declarations<E, F>(
         &self,
         element: E,
@@ -1194,7 +1197,7 @@ impl Stylist {
             matches_user_rules &&
             self.author_styles_enabled == AuthorStylesEnabled::Yes;
 
-        // Step 1: Normal user-agent rules.
+        // Normal user-agent rules.
         if let Some(map) = self.cascade_data.user_agent.cascade_data.normal_rules(pseudo_element) {
             map.get_all_matching_rules(
                 element,
@@ -1206,22 +1209,6 @@ impl Stylist {
             );
         }
 
-        if pseudo_element.is_none() && !only_default_rules {
-            // Step 2: Presentational hints.
-            let length_before_preshints = applicable_declarations.len();
-            element.synthesize_presentational_hints_for_legacy_attributes(
-                context.visited_handling(),
-                applicable_declarations
-            );
-            if applicable_declarations.len() != length_before_preshints {
-                if cfg!(debug_assertions) {
-                    for declaration in &applicable_declarations[length_before_preshints..] {
-                        assert_eq!(declaration.level(), CascadeLevel::PresHints);
-                    }
-                }
-            }
-        }
-
         // NB: the following condition, although it may look somewhat
         // inaccurate, would be equivalent to something like:
         //
@@ -1231,7 +1218,7 @@ impl Stylist {
         //
         // Which may be more what you would probably expect.
         if matches_user_rules {
-            // Step 3a: User normal rules.
+            // User normal rules.
             if let Some(map) = self.cascade_data.user.normal_rules(pseudo_element) {
                 map.get_all_matching_rules(
                     element,
@@ -1244,7 +1231,26 @@ impl Stylist {
             }
         }
 
-        // Step 3b: XBL / Shadow DOM rules.
+        if pseudo_element.is_none() && !only_default_rules {
+            // Presentational hints.
+            //
+            // These go before author rules, but after user rules, see:
+            // https://drafts.csswg.org/css-cascade/#preshint
+            let length_before_preshints = applicable_declarations.len();
+            element.synthesize_presentational_hints_for_legacy_attributes(
+                context.visited_handling(),
+                applicable_declarations
+            );
+            if applicable_declarations.len() != length_before_preshints {
+                if cfg!(debug_assertions) {
+                    for declaration in &applicable_declarations[length_before_preshints..] {
+                        assert_eq!(declaration.level(), CascadeLevel::PresHints);
+                    }
+                }
+            }
+        }
+
+        // XBL / Shadow DOM rules, which are author rules too.
         //
         // TODO(emilio): Cascade order here is wrong for Shadow DOM. In
         // particular, normally document rules override ::slotted() rules, but
@@ -1308,7 +1314,7 @@ impl Stylist {
         });
 
         if matches_author_rules && !only_default_rules && !cut_off_inheritance {
-            // Step 3c: Author normal rules.
+            // Author normal rules.
             if let Some(map) = self.cascade_data.author.normal_rules(pseudo_element) {
                 map.get_all_matching_rules(
                     element,
@@ -1322,7 +1328,7 @@ impl Stylist {
         }
 
         if !only_default_rules {
-            // Step 4: Normal style attributes.
+            // Style attribute ("Normal override declarations").
             if let Some(sa) = style_attribute {
                 applicable_declarations.push(
                     ApplicableDeclarationBlock::from_declarations(
@@ -1332,7 +1338,6 @@ impl Stylist {
                 );
             }
 
-            // Step 5: SMIL override.
             // Declarations from SVG SMIL animation elements.
             if let Some(so) = smil_override {
                 applicable_declarations.push(
@@ -1343,9 +1348,9 @@ impl Stylist {
                 );
             }
 
-            // Step 6: Animations.
-            // The animations sheet (CSS animations, script-generated animations,
-            // and CSS transitions that are no longer tied to CSS markup)
+            // The animations sheet (CSS animations, script-generated
+            // animations, and CSS transitions that are no longer tied to CSS
+            // markup).
             if let Some(anim) = animation_rules.0 {
                 applicable_declarations.push(
                     ApplicableDeclarationBlock::from_declarations(
@@ -1357,13 +1362,12 @@ impl Stylist {
         }
 
         //
-        // Steps 7-10 correspond to !important rules, and are handled during
-        // rule tree insertion.
+        // !important rules are handled during rule tree insertion.
         //
 
         if !only_default_rules {
-            // Step 11: Transitions.
-            // The transitions sheet (CSS transitions that are tied to CSS markup)
+            // The transitions sheet (CSS transitions that are tied to CSS
+            // markup).
             if let Some(anim) = animation_rules.1 {
                 applicable_declarations.push(
                     ApplicableDeclarationBlock::from_declarations(