servo: Merge #20582 - Have CSSOM append rather than replace slot in declaration block (from upsuper:cssom-append); r=emilio

The early return for identical setting in importance matching as well as the comment before `index_to_remove` are removed because the order is web-exposing regardless of whether it's from CSSOM or parsing. e.g. `top: 1px; left: 2px; top: 1px;` is effectively `left: 2px; top: 1px;`,
not `top: 1px; left: 2px;`.

This is patch for [bug 1415330](https://bugzilla.mozilla.org/show_bug.cgi?id=1415330), for spec change in w3c/csswg-drafts#2516. And corresponding test will be added in w3c/web-platform-tests#10354.

Source-Repo: https://github.com/servo/servo
Source-Revision: ccc9d1c4c2877ebc82158b91dc27e1be74fae237

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 9c326db0bca76383e0c47c53baa0fd7f20481130

servo/components/style/properties/declaration_block.rs

@@ -518,53 +518,33 @@ impl PropertyDeclarationBlock {
                 }
 
                 let important = self.declarations_importance.get(i as u32);
-                match (important, importance.important()) {
-                    (false, true) => {}
-
-                    (true, false) => {
-                        // For declarations set from the OM, more-important
-                        // declarations are overridden.
-                        if !matches!(source, DeclarationSource::CssOm) {
-                            return false
-                        }
-                    }
-                    _ => if *slot == declaration {
-                        return false;
-                    }
+                // For declarations from parsing, non-important declarations
+                // shouldn't override existing important one.
+                if important && !importance.important() &&
+                    matches!(source, DeclarationSource::Parsing) {
+                    return true;
                 }
 
-                match source {
-                    // CSSOM preserves the declaration position, and
-                    // overrides importance.
-                    DeclarationSource::CssOm => {
-                        *slot = declaration;
-                        self.declarations_importance.set(i as u32, importance.important());
-                        return true;
-                    }
-                    DeclarationSource::Parsing => {
-                        // As a compatibility hack, specially on Android,
-                        // don't allow to override a prefixed webkit display
-                        // value with an unprefixed version from parsing
-                        // code.
-                        //
-                        // TODO(emilio): Unship.
-                        if let PropertyDeclaration::Display(old_display) = *slot {
-                            use properties::longhands::display::computed_value::T as display;
+                if matches!(source, DeclarationSource::Parsing) {
+                    // As a compatibility hack, specially on Android,
+                    // don't allow to override a prefixed webkit display
+                    // value with an unprefixed version from parsing
+                    // code.
+                    //
+                    // TODO(emilio): Unship.
+                    if let PropertyDeclaration::Display(old_display) = *slot {
+                        use properties::longhands::display::computed_value::T as display;
 
-                            if let PropertyDeclaration::Display(new_display) = declaration {
-                                if display::should_ignore_parsed_value(old_display, new_display) {
-                                    return false;
-                                }
+                        if let PropertyDeclaration::Display(new_display) = declaration {
+                            if display::should_ignore_parsed_value(old_display, new_display) {
+                                return false;
                             }
                         }
-
-                        // NOTE(emilio): We could avoid this and just override for
-                        // properties not affected by logical props, but it's not
-                        // clear it's worth it given the `definitely_new` check.
-                        index_to_remove = Some(i);
-                        break;
                     }
                 }
+
+                index_to_remove = Some(i);
+                break;
             }
 
             if let Some(index) = index_to_remove {