Auto merge of #17972 - heycam:ignore-existing, r=bholley

style: Allow styles to be computed ignoring existing element data.

From https://bugzilla.mozilla.org/show_bug.cgi?id=1384824.

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1 files changed, 3 insertions, 1 deletions

diff --git a/components/style/traversal.rs b/components/style/traversal.rs
index 4640f7c9e0f..39206303ae3 100644
--- a/components/style/traversal.rs
+++ b/components/style/traversal.rs
@@ -391,6 +391,7 @@ pub fn resolve_style<E>(
     context: &mut StyleContext<E>,
     element: E,
     rule_inclusion: RuleInclusion,
+    ignore_existing_style: bool,
 ) -> ElementStyles
 where
     E: TElement,
@@ -398,6 +399,7 @@ where
     use style_resolver::StyleResolverForElement;
 
     debug_assert!(rule_inclusion == RuleInclusion::DefaultOnly ||
+                  ignore_existing_style ||
                   element.borrow_data().map_or(true, |d| !d.has_styles()),
                   "Why are we here?");
     let mut ancestors_requiring_style_resolution = SmallVec::<[E; 16]>::new();
@@ -408,7 +410,7 @@ where
     let mut style = None;
     let mut ancestor = element.traversal_parent();
     while let Some(current) = ancestor {
-        if rule_inclusion == RuleInclusion::All {
+        if rule_inclusion == RuleInclusion::All && !ignore_existing_style {
             if let Some(data) = current.borrow_data() {
                 if let Some(ancestor_style) = data.styles.get_primary() {
                     style = Some(ancestor_style.clone());