--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -146,38 +146,34 @@ DocumentNeedsRestyle(
       }
     } else if (aPseudo == nsCSSPseudoElements::after) {
       if (EffectSet::GetEffectSet(aElement, CSSPseudoElementType::after)) {
         return true;
       }
     }
   }
 
-  if (styleSet->IsServo()) {
-    // For Servo, we need to process the restyle-hint-invalidations first, to
-    // expand LaterSiblings hint, so that we can look whether ancestors need
-    // restyling.
-    ServoRestyleManager* restyleManager =
-      presContext->RestyleManager()->AsServo();
-    restyleManager->ProcessAllPendingAttributeAndStateInvalidations();
-
-    if (!presContext->EffectCompositor()->HasPendingStyleUpdates() &&
-        !aDocument->GetServoRestyleRoot()) {
-      return false;
-    }
-
-    // Then if there is a restyle root, we check if the root is an ancestor of
-    // this content. If it is not, then we don't need to restyle immediately.
-    // Note this is different from Gecko: we only check if any ancestor needs
-    // to restyle _itself_, not descendants, since dirty descendants can be
-    // another subtree.
-    return restyleManager->HasPendingRestyleAncestor(aElement);
-  }
-
-    MOZ_CRASH("old style system disabled");
+  // For Servo, we need to process the restyle-hint-invalidations first, to
+  // expand LaterSiblings hint, so that we can look whether ancestors need
+  // restyling.
+  ServoRestyleManager* restyleManager =
+    presContext->RestyleManager()->AsServo();
+  restyleManager->ProcessAllPendingAttributeAndStateInvalidations();
+
+  if (!presContext->EffectCompositor()->HasPendingStyleUpdates() &&
+      !aDocument->GetServoRestyleRoot()) {
+    return false;
+  }
+
+  // Then if there is a restyle root, we check if the root is an ancestor of
+  // this content. If it is not, then we don't need to restyle immediately.
+  // Note this is different from Gecko: we only check if any ancestor needs
+  // to restyle _itself_, not descendants, since dirty descendants can be
+  // another subtree.
+  return restyleManager->HasPendingRestyleAncestor(aElement);
 }
 
 /**
  * An object that represents the ordered set of properties that are exposed on
  * an nsComputedDOMStyle object and how their computed values can be obtained.
  */
 struct ComputedStyleMap
 {
@@ -420,17 +416,17 @@ uint32_t
 nsComputedDOMStyle::Length()
 {
   uint32_t length = GetComputedStyleMap()->Length();
 
   // Make sure we have up to date style so that we can include custom
   // properties.
   UpdateCurrentStyleSources(false);
   if (mComputedStyle) {
-    length += Servo_GetCustomPropertiesCount(mComputedStyle->AsServo());
+    length += Servo_GetCustomPropertiesCount(mComputedStyle);
   }
 
   ClearCurrentStyleSources();
 
   return length;
 }
 
 css::Rule*
@@ -506,26 +502,17 @@ nsComputedDOMStyle::DoGetComputedStyleNo
   // set.  Using the pres shell from the content also means that any
   // content that's actually *in* a document will get the style from the
   // correct document.
   nsIPresShell* presShell = nsContentUtils::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()) {
+    if (!presShell) {
       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) ||
@@ -560,45 +547,41 @@ nsComputedDOMStyle::DoGetComputedStyleNo
     if (frame) {
       ComputedStyle* result = frame->Style();
       // Don't use the style if it was influenced by pseudo-elements, since then
       // it's not the primary style for this element / pseudo.
       if (!MustReresolveStyle(result)) {
         // The existing style may have animation styles so check if we need to
         // remove them.
         if (aAnimationFlag == eWithoutAnimation) {
-          nsPresContext* presContext = presShell->GetPresContext();
-          MOZ_ASSERT(presContext, "Should have a prescontext if we have a frame");
-          if (presContext && presContext->StyleSet()->IsGecko()) {
-            MOZ_CRASH("old style system disabled");
-          } else {
-            Element* elementOrPseudoElement =
-              EffectCompositor::GetElementToRestyle(aElement, pseudoType);
-            if (!elementOrPseudoElement) {
-              return nullptr;
-            }
-            return presContext->StyleSet()->AsServo()->
-              GetBaseContextForElement(elementOrPseudoElement,
-                                       presContext,
-                                       result->AsServo());
+          nsPresContext* presContext = frame->PresContext();
+          Element* elementOrPseudoElement =
+            EffectCompositor::GetElementToRestyle(aElement, pseudoType);
+          if (!elementOrPseudoElement) {
+            return nullptr;
           }
+          return presContext->StyleSet()->AsServo()->
+            GetBaseContextForElement(elementOrPseudoElement,
+                                     presContext,
+                                     result);
         }
 
         RefPtr<ComputedStyle> ret = result;
         return ret.forget();
       }
     }
   }
 
   // No frame has been created, or we have a pseudo, or we're looking
   // for the default style, so resolve the style ourselves.
 
   nsPresContext* presContext = presShell->GetPresContext();
-  if (!presContext)
+  if (!presContext) {
     return nullptr;
+  }
 
   ServoStyleSet* styleSet = presShell->StyleSet()->AsServo();
 
   StyleRuleInclusion rules = aStyleType == eDefaultOnly
                              ? StyleRuleInclusion::DefaultOnly
                              : StyleRuleInclusion::All;
   RefPtr<ComputedStyle> result =
      styleSet->ResolveStyleLazily(aElement, pseudoType, rules);
@@ -984,27 +967,22 @@ nsComputedDOMStyle::UpdateCurrentStyleSo
                      mPresShell->GetPresContext()->GetUndisplayedRestyleGeneration(),
                    "why should we have flushed style again?");
 
     SetResolvedComputedStyle(Move(resolvedComputedStyle), currentGeneration);
     NS_ASSERTION(mPseudo || !mComputedStyle->HasPseudoElementData(),
                  "should not have pseudo-element data");
   }
 
-  if (mAnimationFlag == eWithoutAnimation) {
-    MOZ_CRASH("old style system disabled");
-  }
-
   // mExposeVisitedStyle is set to true only by testing APIs that
   // require chrome privilege.
   MOZ_ASSERT(!mExposeVisitedStyle || nsContentUtils::IsCallerChrome(),
              "mExposeVisitedStyle set incorrectly");
   if (mExposeVisitedStyle && mComputedStyle->RelevantLinkVisited()) {
-    ComputedStyle *styleIfVisited = mComputedStyle->GetStyleIfVisited();
-    if (styleIfVisited) {
+    if (ComputedStyle* styleIfVisited = mComputedStyle->GetStyleIfVisited()) {
       mComputedStyle = styleIfVisited;
     }
   }
 }
 
 void
 nsComputedDOMStyle::ClearCurrentStyleSources()
 {
@@ -1132,17 +1110,17 @@ nsComputedDOMStyle::IndexedGetter(uint32
 
   uint32_t count =
     Servo_GetCustomPropertiesCount(mComputedStyle);
 
   const uint32_t index = aIndex - length;
   if (index < count) {
     aFound = true;
     nsString varName;
-    Servo_GetCustomPropertyNameAt(mComputedStyle->AsServo(), index, &varName);
+    Servo_GetCustomPropertyNameAt(mComputedStyle, index, &varName);
     aPropName.AssignLiteral("--");
     aPropName.Append(varName);
   } else {
     aFound = false;
   }
 
   ClearCurrentStyleSources();
 }
@@ -7150,18 +7128,17 @@ already_AddRefed<CSSValue>
 nsComputedDOMStyle::DoGetCustomProperty(const nsAString& aPropertyName)
 {
   MOZ_ASSERT(nsCSSProps::IsCustomPropertyName(aPropertyName));
 
   nsString variableValue;
   const nsAString& name = Substring(aPropertyName,
                                     CSS_CUSTOM_NAME_PREFIX_LENGTH);
   bool present =
-    Servo_GetCustomPropertyValue(mComputedStyle->AsServo(), &name,
-                                 &variableValue);
+    Servo_GetCustomPropertyValue(mComputedStyle, &name, &variableValue);
   if (!present) {
     return nullptr;
   }
 
   RefPtr<nsROCSSPrimitiveValue> val = new nsROCSSPrimitiveValue;
   val->SetString(variableValue);
 
   return val.forget();