--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -590,16 +590,26 @@ nsComputedDOMStyle::DoGetStyleContextNoF
   // correct document.
   nsIPresShell *presShell = GetPresShellForContent(aElement);
   bool inDocWithShell = true;
   if (!presShell) {
     inDocWithShell = false;
     presShell = aPresShell;
     if (!presShell)
       return nullptr;
+
+    // In some edge cases, the caller document might be using a different style
+    // backend than the callee. This causes problems because the cached parsed
+    // style attributes in the callee document will be a different format than
+    // the caller expects. Supporting this would be a pain, and we're already
+    // in edge-case-squared, so we just return.
+    if (presShell->GetDocument()->GetStyleBackendType() !=
+        aElement->OwnerDoc()->GetStyleBackendType()) {
+      return nullptr;
+    }
   }
 
   // We do this check to avoid having to add too much special casing of
   // Servo functions we call to explicitly ignore any element data in
   // the tree. (See comment in ServoStyleSet::ResolveStyleLazily.)
   MOZ_RELEASE_ASSERT((aStyleType == eAll && aAnimationFlag == eWithAnimation) ||
                      !aElement->OwnerDoc()->GetBFCacheEntry(),
                      "nsComputedDOMStyle doesn't support getting styles without "