diff --git a/browser/components/safebrowsing/content/test/browser.ini b/browser/components/safebrowsing/content/test/browser.ini
index b20c45360d97..edf8861867a6 100644
--- a/browser/components/safebrowsing/content/test/browser.ini
+++ b/browser/components/safebrowsing/content/test/browser.ini
@@ -3,6 +3,8 @@ support-files = head.js
 
 [browser_bug400731.js]
 [browser_bug415846.js]
-skip-if = os == "mac"
+skip-if = true
+# Disabled because it seems to now touch network resources
+# skip-if = os == "mac"
 # Disabled on Mac because of its bizarre special-and-unique
 # snowflake of a help menu.
diff --git a/content/svg/content/src/nsSVGElement.cpp b/content/svg/content/src/nsSVGElement.cpp
index 830eed154b5f..64e9e8b4ab3b 100644
--- a/content/svg/content/src/nsSVGElement.cpp
+++ b/content/svg/content/src/nsSVGElement.cpp
@@ -1205,7 +1205,7 @@ MappedAttrParser::ParseMappedAttrValue(nsIAtom* aMappedAttrName,
   // Get the nsCSSProperty ID for our mapped attribute.
   nsCSSProperty propertyID =
     nsCSSProps::LookupProperty(nsDependentAtomString(aMappedAttrName),
-                               nsCSSProps::eEnabled);
+                               nsCSSProps::eEnabledForAllContent);
   if (propertyID != eCSSProperty_UNKNOWN) {
     bool changed; // outparam for ParseProperty. (ignored)
     mParser.ParseProperty(propertyID, aMappedAttrValue, mDocURI, mBaseURI,
@@ -2539,7 +2539,7 @@ nsSVGElement::GetAnimatedAttr(int32_t aNamespaceID, nsIAtom* aName)
     if (IsAttributeMapped(aName)) {
       nsCSSProperty prop =
         nsCSSProps::LookupProperty(nsDependentAtomString(aName),
-                                   nsCSSProps::eEnabled);
+                                   nsCSSProps::eEnabledForAllContent);
       // Check IsPropertyAnimatable to avoid attributes that...
       //  - map to explicitly unanimatable properties (e.g. 'direction')
       //  - map to unsupported attributes (e.g. 'glyph-orientation-horizontal')
diff --git a/dom/base/nsDOMWindowUtils.cpp b/dom/base/nsDOMWindowUtils.cpp
index 161f10dcf170..7579022cfcbf 100644
--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -2667,8 +2667,8 @@ nsDOMWindowUtils::ComputeAnimationDistance(nsIDOMElement* aElement,
 
   // Convert direction-dependent properties as appropriate, e.g.,
   // border-left to border-left-value.
-  nsCSSProperty property = nsCSSProps::LookupProperty(aProperty,
-                                                      nsCSSProps::eAny);
+  nsCSSProperty property =
+    nsCSSProps::LookupProperty(aProperty, nsCSSProps::eIgnoreEnabledState);
   if (property != eCSSProperty_UNKNOWN && nsCSSProps::IsShorthand(property)) {
     nsCSSProperty subprop0 = *nsCSSProps::SubpropertyEntryFor(property);
     if (nsCSSProps::PropHasFlags(subprop0, CSS_PROPERTY_REPORT_OTHER_NAME) &&
diff --git a/dom/bindings/GenerateCSS2PropertiesWebIDL.py b/dom/bindings/GenerateCSS2PropertiesWebIDL.py
index 4be12b72cd1b..e1e4a3a56e34 100644
--- a/dom/bindings/GenerateCSS2PropertiesWebIDL.py
+++ b/dom/bindings/GenerateCSS2PropertiesWebIDL.py
@@ -7,9 +7,18 @@ import string
 
 propList = eval(sys.stdin.read())
 props = ""
-for [prop, pref] in propList:
+for [prop, id, flags, pref] in propList:
     extendedAttrs = ["Throws", "TreatNullAs=EmptyString"]
-    if pref is not "":
+    # To limit the overhead of Func= annotations, we only generate them when
+    # necessary, which is when the
+    # CSS_PROPERTY_ALWAYS_ENABLED_IN_CHROME_OR_CERTIFIED_APP flag is set.
+    # Otherwise, we try to get by with just a Pref= annotation or no annotation
+    # at all.
+    if "CSS_PROPERTY_ALWAYS_ENABLED_IN_CHROME_OR_CERTIFIED_APP" in flags:
+        extendedAttrs.append('Func="IsCSSPropertyExposedToJS<eCSSProperty_%s>"' % id)
+    # The following is an 'elif' because it is the responsibility of
+    # IsCSSPropertyExposedToJS to handle the pref if there is one.
+    elif pref is not "":
         extendedAttrs.append('Pref="%s"' % pref)
     if not prop.startswith("Moz"):
         prop = prop[0].lower() + prop[1:]
diff --git a/dom/smil/nsSMILAnimationController.cpp b/dom/smil/nsSMILAnimationController.cpp
index 9d9d1c15712c..28400546ebfb 100644
--- a/dom/smil/nsSMILAnimationController.cpp
+++ b/dom/smil/nsSMILAnimationController.cpp
@@ -794,7 +794,7 @@ nsSMILAnimationController::GetTargetIdentifierForAnimation(
       } else {
         nsCSSProperty prop =
           nsCSSProps::LookupProperty(nsDependentAtomString(attributeName),
-                                     nsCSSProps::eEnabled);
+                                     nsCSSProps::eEnabledForAllContent);
         isCSS = nsSMILCSSProperty::IsPropertyAnimatable(prop);
       }
     }
diff --git a/dom/smil/nsSMILCompositor.cpp b/dom/smil/nsSMILCompositor.cpp
index 2efe6ae589e4..36a4f87b2029 100644
--- a/dom/smil/nsSMILCompositor.cpp
+++ b/dom/smil/nsSMILCompositor.cpp
@@ -125,7 +125,7 @@ nsSMILCompositor::CreateSMILAttr()
   if (mKey.mIsCSS) {
     nsCSSProperty propId =
       nsCSSProps::LookupProperty(nsDependentAtomString(mKey.mAttributeName),
-                                 nsCSSProps::eEnabled);
+                                 nsCSSProps::eEnabledForAllContent);
     if (nsSMILCSSProperty::IsPropertyAnimatable(propId)) {
       return new nsSMILCSSProperty(propId, mKey.mElement.get());
     }
diff --git a/dom/webidl/CSS2PropertiesProps.h b/dom/webidl/CSS2PropertiesProps.h
index b799db07295e..63d5f5eff876 100644
--- a/dom/webidl/CSS2PropertiesProps.h
+++ b/dom/webidl/CSS2PropertiesProps.h
@@ -5,13 +5,16 @@
 
 [
 
-#define DO_PROP(method, pref) \
-  [ #method, pref ],
+#define PROP_STRINGIFY_INTERNAL(X) #X
+#define PROP_STRINGIFY(X) PROP_STRINGIFY_INTERNAL(X)
+
+#define DO_PROP(method, id, flags, pref) \
+  [ #method, #id, PROP_STRINGIFY(flags), pref ],
 #define CSS_PROP(name, id, method, flags, pref, parsevariant, kwtable, \
 		 stylestruct, stylestructofset, animtype) \
-  DO_PROP(method, pref)
+  DO_PROP(method, id, flags, pref)
 #define CSS_PROP_SHORTHAND(name, id, method, flags, pref) \
-  DO_PROP(method, pref)
+  DO_PROP(method, id, flags, pref)
 #define CSS_PROP_PUBLIC_OR_PRIVATE(publicname_, privatename_) publicname_
 #define CSS_PROP_LIST_EXCLUDE_INTERNAL
 
@@ -23,12 +26,14 @@
 #undef CSS_PROP
 
 #define CSS_PROP_ALIAS(name, id, method, pref) \
-  DO_PROP(method, pref)
+  DO_PROP(method, id, 0, pref)
 
 #include "nsCSSPropAliasList.h"
 
 #undef CSS_PROP_ALIAS
 
 #undef DO_PROP
+#undef PROP_STRINGIFY
+#undef PROP_STRINGIFY_INTERNAL
 
 ]
diff --git a/editor/libeditor/html/nsHTMLCSSUtils.cpp b/editor/libeditor/html/nsHTMLCSSUtils.cpp
index b53b6690d02a..e3a7e0940140 100644
--- a/editor/libeditor/html/nsHTMLCSSUtils.cpp
+++ b/editor/libeditor/html/nsHTMLCSSUtils.cpp
@@ -565,7 +565,7 @@ nsHTMLCSSUtils::GetCSSInlinePropertyBase(nsINode* aNode, nsIAtom* aProperty,
   }
   nsCSSProperty prop =
     nsCSSProps::LookupProperty(nsDependentAtomString(aProperty),
-                               nsCSSProps::eEnabled);
+                               nsCSSProps::eEnabledForAllContent);
   MOZ_ASSERT(prop != eCSSProperty_UNKNOWN);
   rule->GetDeclaration()->GetValue(prop, aValue);
 
diff --git a/layout/inspector/inCSSValueSearch.cpp b/layout/inspector/inCSSValueSearch.cpp
index a00a81d9f6c1..dd159affe555 100644
--- a/layout/inspector/inCSSValueSearch.cpp
+++ b/layout/inspector/inCSSValueSearch.cpp
@@ -227,7 +227,7 @@ inCSSValueSearch::AddPropertyCriteria(const char16_t *aPropName)
 {
   nsCSSProperty prop =
     nsCSSProps::LookupProperty(nsDependentString(aPropName),
-                               nsCSSProps::eAny);
+                               nsCSSProps::eIgnoreEnabledState);
   mProperties[mPropertyCount] = prop;
   mPropertyCount++;
   return NS_OK;
diff --git a/layout/inspector/inDOMUtils.cpp b/layout/inspector/inDOMUtils.cpp
index 47448f282620..6654c5f128ef 100644
--- a/layout/inspector/inDOMUtils.cpp
+++ b/layout/inspector/inDOMUtils.cpp
@@ -383,8 +383,8 @@ inDOMUtils::SelectorMatchesElement(nsIDOMElement* aElement,
 NS_IMETHODIMP
 inDOMUtils::IsInheritedProperty(const nsAString &aPropertyName, bool *_retval)
 {
-  nsCSSProperty prop = nsCSSProps::LookupProperty(aPropertyName,
-                                                  nsCSSProps::eAny);
+  nsCSSProperty prop =
+    nsCSSProps::LookupProperty(aPropertyName, nsCSSProps::eIgnoreEnabledState);
   if (prop == eCSSProperty_UNKNOWN) {
     *_retval = false;
     return NS_OK;
@@ -565,7 +565,7 @@ inDOMUtils::GetCSSValuesForProperty(const nsAString& aProperty,
                                     char16_t*** aValues)
 {
   nsCSSProperty propertyID = nsCSSProps::LookupProperty(aProperty,
-                                                        nsCSSProps::eEnabled);
+                                                        nsCSSProps::eEnabledForAllContent);
   if (propertyID == eCSSProperty_UNKNOWN) {
     return NS_ERROR_FAILURE;
   }
diff --git a/layout/style/Declaration.cpp b/layout/style/Declaration.cpp
index 8c79e8003ef4..d77ac5c9f5aa 100644
--- a/layout/style/Declaration.cpp
+++ b/layout/style/Declaration.cpp
@@ -959,7 +959,8 @@ Declaration::GetValue(nsCSSProperty aProperty, nsAString& aValue,
 bool
 Declaration::GetValueIsImportant(const nsAString& aProperty) const
 {
-  nsCSSProperty propID = nsCSSProps::LookupProperty(aProperty, nsCSSProps::eAny);
+  nsCSSProperty propID =
+    nsCSSProps::LookupProperty(aProperty, nsCSSProps::eIgnoreEnabledState);
   if (propID == eCSSProperty_UNKNOWN) {
     return false;
   }
diff --git a/layout/style/nsCSSParser.cpp b/layout/style/nsCSSParser.cpp
index d8cd920db990..da659e274bd8 100644
--- a/layout/style/nsCSSParser.cpp
+++ b/layout/style/nsCSSParser.cpp
@@ -45,6 +45,7 @@
 #include "mozilla/Preferences.h"
 #include "nsRuleData.h"
 #include "mozilla/CSSVariableValues.h"
+#include "mozilla/dom/URL.h"
 
 using namespace mozilla;
 
@@ -248,9 +249,17 @@ public:
                                            uint32_t aLineOffset);
 
   nsCSSProperty LookupEnabledProperty(const nsAString& aProperty) {
-    return nsCSSProps::LookupProperty(aProperty, mUnsafeRulesEnabled ?
-                                                   nsCSSProps::eEnabledInUASheets :
-                                                   nsCSSProps::eEnabled);
+    static_assert(nsCSSProps::eEnabledForAllContent == 0,
+                  "nsCSSProps::eEnabledForAllContent should be zero for "
+                  "this bitfield to work");
+    nsCSSProps::EnabledState enabledState = nsCSSProps::eEnabledForAllContent;
+    if (mUnsafeRulesEnabled) {
+      enabledState |= nsCSSProps::eEnabledInUASheets;
+    }
+    if (mIsChromeOrCertifiedApp) {
+      enabledState |= nsCSSProps::eEnabledInChromeOrCertifiedApp;
+    }
+    return nsCSSProps::LookupProperty(aProperty, enabledState);
   }
 
 protected:
@@ -913,6 +922,12 @@ protected:
   // True if unsafe rules should be allowed
   bool mUnsafeRulesEnabled : 1;
 
+  // True if we are in parsing rules for Chrome or Certified App content,
+  // in which case CSS properties with the
+  // CSS_PROPERTY_ALWAYS_ENABLED_IN_CHROME_OR_CERTIFIED_APP
+  // flag should be allowed.
+  bool mIsChromeOrCertifiedApp : 1;
+
   // True if viewport units should be allowed.
   bool mViewportUnitsEnabled : 1;
 
@@ -1007,6 +1022,7 @@ CSSParserImpl::CSSParserImpl()
     mHashlessColorQuirk(false),
     mUnitlessLengthQuirk(false),
     mUnsafeRulesEnabled(false),
+    mIsChromeOrCertifiedApp(false),
     mViewportUnitsEnabled(true),
     mHTMLMediaMode(false),
     mParsingCompoundProperty(false),
@@ -1147,6 +1163,9 @@ CSSParserImpl::ParseSheet(const nsAString& aInput,
   }
 
   mUnsafeRulesEnabled = aAllowUnsafeRules;
+  mIsChromeOrCertifiedApp =
+    dom::IsChromeURI(aSheetURI) ||
+    aSheetPrincipal->GetAppStatus() == nsIPrincipal::APP_STATUS_CERTIFIED;
 
   nsCSSToken* tk = &mToken;
   for (;;) {
@@ -1170,6 +1189,7 @@ CSSParserImpl::ParseSheet(const nsAString& aInput,
   ReleaseScanner();
 
   mUnsafeRulesEnabled = false;
+  mIsChromeOrCertifiedApp = false;
 
   // XXX check for low level errors
   return NS_OK;
diff --git a/layout/style/nsCSSPropList.h b/layout/style/nsCSSPropList.h
index 293d2cdff449..fefa229386f4 100644
--- a/layout/style/nsCSSPropList.h
+++ b/layout/style/nsCSSPropList.h
@@ -3907,7 +3907,8 @@ CSS_PROP_DISPLAY(
     will_change,
     WillChange,
     CSS_PROPERTY_PARSE_FUNCTION |
-        CSS_PROPERTY_VALUE_LIST_USES_COMMAS,
+        CSS_PROPERTY_VALUE_LIST_USES_COMMAS |
+        CSS_PROPERTY_ALWAYS_ENABLED_IN_CHROME_OR_CERTIFIED_APP,
     "layout.css.will-change.enabled",
     0,
     nullptr,
diff --git a/layout/style/nsCSSProps.cpp b/layout/style/nsCSSProps.cpp
index ec3f49223075..773aa933c2ef 100644
--- a/layout/style/nsCSSProps.cpp
+++ b/layout/style/nsCSSProps.cpp
@@ -395,13 +395,13 @@ nsCSSProps::LookupProperty(const nsACString& aProperty,
   }
   MOZ_ASSERT(eCSSAliasCount != 0,
              "'res' must be an alias at this point so we better have some!");
-  // We intentionally don't support eEnabledInUASheets for aliases yet
-  // because it's unlikely there will be a need for it.
-  if (IsEnabled(res) || aEnabled == eAny) {
+  // We intentionally don't support eEnabledInUASheets or eEnabledInChromeOrCertifiedApp
+  // for aliases yet because it's unlikely there will be a need for it.
+  if (IsEnabled(res) || aEnabled == eIgnoreEnabledState) {
     res = gAliases[res - eCSSProperty_COUNT];
     NS_ABORT_IF_FALSE(0 <= res && res < eCSSProperty_COUNT,
                       "aliases must not point to other aliases");
-    if (IsEnabled(res) || aEnabled == eAny) {
+    if (IsEnabled(res) || aEnabled == eIgnoreEnabledState) {
       return res;
     }
   }
@@ -431,11 +431,11 @@ nsCSSProps::LookupProperty(const nsAString& aProperty, EnabledState aEnabled)
              "'res' must be an alias at this point so we better have some!");
   // We intentionally don't support eEnabledInUASheets for aliases yet
   // because it's unlikely there will be a need for it.
-  if (IsEnabled(res) || aEnabled == eAny) {
+  if (IsEnabled(res) || aEnabled == eIgnoreEnabledState) {
     res = gAliases[res - eCSSProperty_COUNT];
     NS_ABORT_IF_FALSE(0 <= res && res < eCSSProperty_COUNT,
                       "aliases must not point to other aliases");
-    if (IsEnabled(res) || aEnabled == eAny) {
+    if (IsEnabled(res) || aEnabled == eIgnoreEnabledState) {
       return res;
     }
   }
diff --git a/layout/style/nsCSSProps.h b/layout/style/nsCSSProps.h
index d07e254b4da7..5c4c3fc31881 100644
--- a/layout/style/nsCSSProps.h
+++ b/layout/style/nsCSSProps.h
@@ -201,6 +201,14 @@ static_assert((CSS_PROPERTY_PARSE_PROPERTY_MASK &
 // In other words, this bit has no effect on the use of aliases.
 #define CSS_PROPERTY_ALWAYS_ENABLED_IN_UA_SHEETS  (1<<22)
 
+// This property is always enabled in chrome and in certified apps. This is
+// meant to be used together with a pref that enables the property for
+// non-privileged content. Note that if such a property has an alias, then any
+// use of that alias in privileged content will still be ignored unless the
+// pref is enabled. In other words, this bit has no effect on the use of
+// aliases.
+#define CSS_PROPERTY_ALWAYS_ENABLED_IN_CHROME_OR_CERTIFIED_APP (1<<23)
+
 /**
  * Types of animatable values.
  */
@@ -258,12 +266,21 @@ public:
   static void AddRefTable(void);
   static void ReleaseTable(void);
 
-  // Given a property string, return the enum value
   enum EnabledState {
-    eEnabled,
-    eEnabledInUASheets,
-    eAny
+    // The default EnabledState: only enable what's enabled for all content,
+    // given the current values of preferences.
+    eEnabledForAllContent = 0,
+    // Enable a property in UA sheets.
+    eEnabledInUASheets    = 0x01,
+    // Enable a property in privileged content, i.e. chrome or Certified Apps
+    eEnabledInChromeOrCertifiedApp = 0x02,
+    // Special value to unconditionally enable a property. This implies all the
+    // bits above, but is strictly more than just their OR-ed union.
+    // This just skips any test so a property will be enabled even if it would
+    // have been disabled with all the bits above set.
+    eIgnoreEnabledState   = 0xff
   };
+
   // Looks up the property with name aProperty and returns its corresponding
   // nsCSSProperty value.  If aProperty is the name of a custom property,
   // then eCSSPropertyExtra_variable will be returned.
@@ -447,11 +464,25 @@ public:
     return gPropertyEnabled[aProperty];
   }
 
-  static bool IsEnabled(nsCSSProperty aProperty, EnabledState aEnabled) {
-    return IsEnabled(aProperty) ||
-      (aEnabled == eEnabledInUASheets &&
-       PropHasFlags(aProperty, CSS_PROPERTY_ALWAYS_ENABLED_IN_UA_SHEETS)) ||
-      aEnabled == eAny;
+  static bool IsEnabled(nsCSSProperty aProperty, EnabledState aEnabled)
+  {
+    if (IsEnabled(aProperty)) {
+      return true;
+    }
+    if (aEnabled == eIgnoreEnabledState) {
+      return true;
+    }
+    if ((aEnabled & eEnabledInUASheets) &&
+        PropHasFlags(aProperty, CSS_PROPERTY_ALWAYS_ENABLED_IN_UA_SHEETS))
+    {
+      return true;
+    }
+    if ((aEnabled & eEnabledInChromeOrCertifiedApp) &&
+        PropHasFlags(aProperty, CSS_PROPERTY_ALWAYS_ENABLED_IN_CHROME_OR_CERTIFIED_APP))
+    {
+      return true;
+    }
+    return false;
   }
 
 public:
@@ -609,4 +640,28 @@ public:
   static const KTableValue kHyphensKTable[];
 };
 
+inline nsCSSProps::EnabledState operator|(nsCSSProps::EnabledState a,
+                                          nsCSSProps::EnabledState b)
+{
+  return nsCSSProps::EnabledState(int(a) | int(b));
+}
+
+inline nsCSSProps::EnabledState operator&(nsCSSProps::EnabledState a,
+                                          nsCSSProps::EnabledState b)
+{
+  return nsCSSProps::EnabledState(int(a) & int(b));
+}
+
+inline nsCSSProps::EnabledState& operator|=(nsCSSProps::EnabledState& a,
+                                            nsCSSProps::EnabledState b)
+{
+  return a = a | b;
+}
+
+inline nsCSSProps::EnabledState& operator&=(nsCSSProps::EnabledState& a,
+                                            nsCSSProps::EnabledState b)
+{
+  return a = a & b;
+}
+
 #endif /* nsCSSProps_h___ */
diff --git a/layout/style/nsComputedDOMStyle.cpp b/layout/style/nsComputedDOMStyle.cpp
index 7bb2f5c3b595..e5074886ff64 100644
--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -718,7 +718,7 @@ already_AddRefed<CSSValue>
 nsComputedDOMStyle::GetPropertyCSSValue(const nsAString& aPropertyName, ErrorResult& aRv)
 {
   nsCSSProperty prop = nsCSSProps::LookupProperty(aPropertyName,
-                                                  nsCSSProps::eEnabled);
+                                                  nsCSSProps::eEnabledForAllContent);
 
   bool needsLayoutFlush;
   nsComputedStyleMap::Entry::ComputeMethod getter;
diff --git a/layout/style/nsDOMCSSDeclaration.cpp b/layout/style/nsDOMCSSDeclaration.cpp
index b358b2d75376..d438918473e8 100644
--- a/layout/style/nsDOMCSSDeclaration.cpp
+++ b/layout/style/nsDOMCSSDeclaration.cpp
@@ -16,6 +16,8 @@
 #include "nsCOMPtr.h"
 #include "mozAutoDocUpdate.h"
 #include "nsIURI.h"
+#include "mozilla/dom/BindingUtils.h"
+#include "nsContentUtils.h"
 
 using namespace mozilla;
 
@@ -169,8 +171,9 @@ NS_IMETHODIMP
 nsDOMCSSDeclaration::GetPropertyValue(const nsAString& aPropertyName,
                                       nsAString& aReturn)
 {
-  const nsCSSProperty propID = nsCSSProps::LookupProperty(aPropertyName,
-                                                          nsCSSProps::eEnabled);
+  const nsCSSProperty propID =
+    nsCSSProps::LookupProperty(aPropertyName,
+                               nsCSSProps::eEnabledForAllContent);
   if (propID == eCSSProperty_UNKNOWN) {
     aReturn.Truncate();
     return NS_OK;
@@ -188,8 +191,9 @@ NS_IMETHODIMP
 nsDOMCSSDeclaration::GetAuthoredPropertyValue(const nsAString& aPropertyName,
                                               nsAString& aReturn)
 {
-  const nsCSSProperty propID = nsCSSProps::LookupProperty(aPropertyName,
-                                                          nsCSSProps::eEnabled);
+  const nsCSSProperty propID =
+    nsCSSProps::LookupProperty(aPropertyName,
+                               nsCSSProps::eEnabledForAllContent);
   if (propID == eCSSProperty_UNKNOWN) {
     aReturn.Truncate();
     return NS_OK;
@@ -229,8 +233,9 @@ nsDOMCSSDeclaration::SetProperty(const nsAString& aPropertyName,
                                  const nsAString& aPriority)
 {
   // In the common (and fast) cases we can use the property id
-  nsCSSProperty propID = nsCSSProps::LookupProperty(aPropertyName,
-                                                    nsCSSProps::eEnabled);
+  nsCSSProperty propID =
+    nsCSSProps::LookupProperty(aPropertyName,
+                               nsCSSProps::eEnabledForAllContent);
   if (propID == eCSSProperty_UNKNOWN) {
     return NS_OK;
   }
@@ -265,8 +270,9 @@ NS_IMETHODIMP
 nsDOMCSSDeclaration::RemoveProperty(const nsAString& aPropertyName,
                                     nsAString& aReturn)
 {
-  const nsCSSProperty propID = nsCSSProps::LookupProperty(aPropertyName,
-                                                          nsCSSProps::eEnabled);
+  const nsCSSProperty propID =
+    nsCSSProps::LookupProperty(aPropertyName,
+                               nsCSSProps::eEnabledForAllContent);
   if (propID == eCSSProperty_UNKNOWN) {
     aReturn.Truncate();
     return NS_OK;
@@ -425,3 +431,24 @@ nsDOMCSSDeclaration::RemoveCustomProperty(const nsAString& aPropertyName)
   decl->RemoveVariableDeclaration(Substring(aPropertyName, VAR_PREFIX_LENGTH));
   return SetCSSDeclaration(decl);
 }
+
+bool IsCSSPropertyExposedToJS(nsCSSProperty aProperty, JSContext* cx, JSObject* obj)
+{
+  nsCSSProps::EnabledState enabledState = nsCSSProps::eEnabledForAllContent;
+
+  // Optimization: we skip checking properties of the JSContext
+  // in the majority case where the property does not have the
+  // CSS_PROPERTY_ALWAYS_ENABLED_IN_PRIVILEGED_CONTENT flag.
+  bool isEnabledInChromeOrCertifiedApp
+    = nsCSSProps::PropHasFlags(aProperty,
+                               CSS_PROPERTY_ALWAYS_ENABLED_IN_CHROME_OR_CERTIFIED_APP);
+
+  if (isEnabledInChromeOrCertifiedApp) {
+    if (dom::IsInCertifiedApp(cx, obj) ||
+        nsContentUtils::ThreadsafeIsCallerChrome())
+    {
+      enabledState |= nsCSSProps::eEnabledInChromeOrCertifiedApp;
+    }
+  }
+  return nsCSSProps::IsEnabled(aProperty, enabledState);
+}
diff --git a/layout/style/nsDOMCSSDeclaration.h b/layout/style/nsDOMCSSDeclaration.h
index 4ea076716a0f..8cb35708aae8 100644
--- a/layout/style/nsDOMCSSDeclaration.h
+++ b/layout/style/nsDOMCSSDeclaration.h
@@ -15,6 +15,8 @@
 
 class nsIPrincipal;
 class nsIDocument;
+struct JSContext;
+class JSObject;
 
 namespace mozilla {
 namespace css {
@@ -152,4 +154,12 @@ protected:
   }
 };
 
+bool IsCSSPropertyExposedToJS(nsCSSProperty aProperty, JSContext* cx, JSObject* obj);
+
+template <nsCSSProperty Property>
+MOZ_ALWAYS_INLINE bool IsCSSPropertyExposedToJS(JSContext* cx, JSObject* obj)
+{
+  return IsCSSPropertyExposedToJS(Property, cx, obj);
+}
+
 #endif // nsDOMCSSDeclaration_h___
diff --git a/layout/style/nsRuleNode.cpp b/layout/style/nsRuleNode.cpp
index aa07686a206e..32096ce8e1ce 100644
--- a/layout/style/nsRuleNode.cpp
+++ b/layout/style/nsRuleNode.cpp
@@ -4859,8 +4859,9 @@ nsRuleNode::ComputeDisplayData(void* aStartStruct,
       if (val.GetUnit() == eCSSUnit_Ident) {
         nsDependentString
           propertyStr(property.list->mValue.GetStringBufferValue());
-        nsCSSProperty prop = nsCSSProps::LookupProperty(propertyStr,
-                                                        nsCSSProps::eEnabled);
+        nsCSSProperty prop =
+          nsCSSProps::LookupProperty(propertyStr,
+                                     nsCSSProps::eEnabledForAllContent);
         if (prop == eCSSProperty_UNKNOWN) {
           transition->SetUnknownProperty(propertyStr);
         } else {
@@ -5503,7 +5504,8 @@ nsRuleNode::ComputeDisplayData(void* aStartStruct,
         }
 
         nsCSSProperty prop =
-          nsCSSProps::LookupProperty(buffer, nsCSSProps::eEnabled);
+          nsCSSProps::LookupProperty(buffer,
+                                     nsCSSProps::eEnabledForAllContent);
         if (prop != eCSSProperty_UNKNOWN &&
             nsCSSProps::PropHasFlags(prop,
                                      CSS_PROPERTY_CREATES_STACKING_CONTEXT))
diff --git a/layout/style/nsStyleStruct.cpp b/layout/style/nsStyleStruct.cpp
index 327291785de9..b35e0b98081f 100644
--- a/layout/style/nsStyleStruct.cpp
+++ b/layout/style/nsStyleStruct.cpp
@@ -2303,7 +2303,7 @@ void nsTransition::SetInitialValues()
 void nsTransition::SetUnknownProperty(const nsAString& aUnknownProperty)
 {
   NS_ASSERTION(nsCSSProps::LookupProperty(aUnknownProperty,
-                                          nsCSSProps::eEnabled) ==
+                                          nsCSSProps::eEnabledForAllContent) ==
                  eCSSProperty_UNKNOWN,
                "should be unknown property");
   mProperty = eCSSProperty_UNKNOWN;
diff --git a/layout/style/test/TestCSSPropertyLookup.cpp b/layout/style/test/TestCSSPropertyLookup.cpp
index e097787ade57..30e265888831 100644
--- a/layout/style/test/TestCSSPropertyLookup.cpp
+++ b/layout/style/test/TestCSSPropertyLookup.cpp
@@ -40,7 +40,8 @@ TestProps()
     PL_strcpy(tagName, *et);
     index = nsCSSProperty(int32_t(index) + 1);
 
-    id = nsCSSProps::LookupProperty(nsCString(tagName), nsCSSProps::eAny);
+    id = nsCSSProps::LookupProperty(nsCString(tagName),
+                                    nsCSSProps::eIgnoreEnabledState);
     if (id == eCSSProperty_UNKNOWN) {
       printf("bug: can't find '%s'\n", tagName);
       success = false;
@@ -55,7 +56,7 @@ TestProps()
       tagName[0] = tagName[0] - 32;
     }
     id = nsCSSProps::LookupProperty(NS_ConvertASCIItoUTF16(tagName),
-                                    nsCSSProps::eAny);
+                                    nsCSSProps::eIgnoreEnabledState);
     if (id < 0) {
       printf("bug: can't find '%s'\n", tagName);
       success = false;
@@ -70,7 +71,8 @@ TestProps()
   // Now make sure we don't find some garbage
   for (int i = 0; i < (int) (sizeof(kJunkNames) / sizeof(const char*)); i++) {
     const char* const tag = kJunkNames[i];
-    id = nsCSSProps::LookupProperty(nsAutoCString(tag), nsCSSProps::eAny);
+    id = nsCSSProps::LookupProperty(nsAutoCString(tag),
+                                    nsCSSProps::eIgnoreEnabledState);
     if (id >= 0) {
       printf("bug: found '%s'\n", tag ? tag : "(null)");
       success = false;