--- a/content/base/src/DirectionalityUtils.cpp
+++ b/content/base/src/DirectionalityUtils.cpp
@@ -230,20 +230,20 @@ typedef mozilla::dom::Element Element;
  * test for it separately, e.g. with DoesNotAffectDirectionOfAncestors.
  * It *does* include textarea, because even if a textarea has dir=auto, it has
  * unicode-bidi: plaintext and is handled automatically in bidi resolution.
  */
 static bool
 DoesNotParticipateInAutoDirection(const Element* aElement)
 {
   nsINodeInfo* nodeInfo = aElement->NodeInfo();
-  return (aElement->IsHTML() &&
-          (nodeInfo->Equals(nsGkAtoms::script) ||
-           nodeInfo->Equals(nsGkAtoms::style) ||
-           nodeInfo->Equals(nsGkAtoms::textarea)));
+  return (!aElement->IsHTML() ||
+          nodeInfo->Equals(nsGkAtoms::script) ||
+          nodeInfo->Equals(nsGkAtoms::style) ||
+          nodeInfo->Equals(nsGkAtoms::textarea));
 }
 
 /**
  * Returns true if aElement is one of the element whose text content should not
  * affect the direction of ancestors with dir=auto (though it may affect its own
  * direction, e.g. <bdi>)
  */
 static bool
@@ -637,31 +637,41 @@ WalkDescendantsResetAutoDirection(Elemen
     }
     child = child->GetNextNode(aElement);
   }
 }
 
 void
 WalkDescendantsSetDirAuto(Element* aElement, bool aNotify)
 {
-  bool setAncestorDirAutoFlag =
+  if (!DoesNotParticipateInAutoDirection(aElement) &&
+      !aElement->IsHTML(nsGkAtoms::bdi)) {
+
+    bool setAncestorDirAutoFlag =
 #ifdef DEBUG
-    true;
+      true;
 #else
-    !aElement->AncestorHasDirAuto();
+      !aElement->AncestorHasDirAuto();
 #endif
 
-  if (setAncestorDirAutoFlag) {
-    nsIContent* child = aElement->GetFirstChild();
-    while (child) {
-      MOZ_ASSERT(!aElement->AncestorHasDirAuto() ||
-                 child->AncestorHasDirAuto(),
-                 "AncestorHasDirAuto set on node but not its children");
-      child->SetHasDirAuto();
-      child = child->GetNextNode(aElement);
+    if (setAncestorDirAutoFlag) {
+      nsIContent* child = aElement->GetFirstChild();
+      while (child) {
+        if (child->IsElement() &&
+            DoesNotAffectDirectionOfAncestors(child->AsElement())) {
+          child = child->GetNextNonChildNode(aElement);
+          continue;
+        }
+
+        MOZ_ASSERT(!aElement->AncestorHasDirAuto() ||
+                   child->AncestorHasDirAuto(),
+                   "AncestorHasDirAuto set on node but not its children");
+        child->SetHasDirAuto();
+        child = child->GetNextNode(aElement);
+      }
     }
   }
 
   nsINode* textNode = WalkDescendantsSetDirectionFromText(aElement, aNotify);
   if (textNode) {
     nsTextNodeDirectionalityMap::AddEntryToMap(textNode, aElement);
   }
 }