--- a/layout/style/SheetParsingMode.h
+++ b/layout/style/SheetParsingMode.h
@@ -19,25 +19,35 @@ namespace css {
  * Author sheets are the normal case: styles embedded in or linked
  * from HTML pages.  They are also the most restricted.
  *
  * User sheets can do anything author sheets can do, and also get
  * access to a few CSS extensions that are not yet suitable for
  * exposure on the public Web, but are very useful for expressing
  * user style overrides, such as @-moz-document rules.
  *
+ * XXX: eUserSheetFeatures was added in bug 1035091, but some patches in
+ * that bug never landed to use this enum value. Currently, all the features
+ * in user sheet are also available in author sheet.
+ *
  * Agent sheets have access to all author- and user-sheet features
  * plus more extensions that are necessary for internal use but,
  * again, not yet suitable for exposure on the public Web.  Some of
  * these are outright unsafe to expose; in particular, incorrect
  * styling of anonymous box pseudo-elements can violate layout
  * invariants.
+ *
+ * Agent sheets that do not use any unsafe rules could use
+ * eSafeAgentSheetFeatures when creating the sheet. This enum value allows
+ * Servo backend to recognize the sheets as the agent level, but Gecko
+ * backend will parse it under _author_ level.
  */
 enum SheetParsingMode {
   eAuthorSheetFeatures = 0,
   eUserSheetFeatures,
-  eAgentSheetFeatures
+  eAgentSheetFeatures,
+  eSafeAgentSheetFeatures,
 };
 
 } // namespace css
 } // namespace mozilla
 
 #endif // mozilla_css_SheetParsingMode_h

--- a/layout/style/nsCSSParser.cpp
+++ b/layout/style/nsCSSParser.cpp
@@ -358,30 +358,26 @@ public:
                                            uint32_t aLineOffset);
 
   bool AgentRulesEnabled() const {
     return mParsingMode == css::eAgentSheetFeatures;
   }
   bool ChromeRulesEnabled() const {
     return mIsChrome;
   }
-  bool UserRulesEnabled() const {
-    return mParsingMode == css::eAgentSheetFeatures ||
-           mParsingMode == css::eUserSheetFeatures;
-  }
 
   CSSEnabledState EnabledState() const {
     static_assert(int(CSSEnabledState::eForAllContent) == 0,
                   "CSSEnabledState::eForAllContent should be zero for "
                   "this bitfield to work");
     CSSEnabledState enabledState = CSSEnabledState::eForAllContent;
     if (AgentRulesEnabled()) {
       enabledState |= CSSEnabledState::eInUASheets;
     }
-    if (mIsChrome) {
+    if (ChromeRulesEnabled()) {
       enabledState |= CSSEnabledState::eInChrome;
     }
     return enabledState;
   }
 
   nsCSSPropertyID LookupEnabledProperty(const nsAString& aProperty) {
     return nsCSSProps::LookupProperty(aProperty, EnabledState());
   }

--- a/layout/style/nsLayoutStylesheetCache.cpp
+++ b/layout/style/nsLayoutStylesheetCache.cpp
@@ -74,17 +74,17 @@ nsLayoutStylesheetCache::Observe(nsISupp
 }
 
 StyleSheet*
 nsLayoutStylesheetCache::ScrollbarsSheet()
 {
   if (!mScrollbarsSheet) {
     // Scrollbars don't need access to unsafe rules
     LoadSheetURL("chrome://global/skin/scrollbars.css",
-                 &mScrollbarsSheet, eAuthorSheetFeatures, eCrash);
+                 &mScrollbarsSheet, eSafeAgentSheetFeatures, eCrash);
   }
 
   return mScrollbarsSheet;
 }
 
 StyleSheet*
 nsLayoutStylesheetCache::FormsSheet()
 {