--- a/layout/style/nsCSSProps.cpp
+++ b/layout/style/nsCSSProps.cpp
@@ -18,16 +18,19 @@
 #include "nsThemeConstants.h"  // For system widget appearance types
 
 #include "mozilla/dom/AnimationEffectReadOnlyBinding.h" // for PlaybackDirection
 #include "mozilla/LookAndFeel.h" // for system colors
 
 #include "nsString.h"
 #include "nsStaticNameTable.h"
 
+#include "mozilla/CSSProperty.h"
+#include "mozilla/CSSVariableRegistration.h"
+#include "mozilla/CSSVariableRegistrations.h"
 #include "mozilla/Preferences.h"
 
 using namespace mozilla;
 
 typedef nsCSSProps::KTableEntry KTableEntry;
 
 // By wrapping internal-only properties in this macro, we are not
 // exposing them in the CSSOM. Since currently it is not necessary to
@@ -602,16 +605,38 @@ nsCSSPropertyID
 nsCSSProps::LookupPropertyByIDLName(const nsAString& aPropertyIDLName,
                                     EnabledState aEnabled)
 {
   MOZ_ASSERT(gPropertyIDLNameTable, "no lookup table, needs addref");
   return LookupPropertyByIDLName(NS_ConvertUTF16toUTF8(aPropertyIDLName),
                                  aEnabled);
 }
 
+/* static */ mozilla::CSSProperty
+nsCSSProps::LookupCustomProperty(const CSSVariableRegistrations* aRegistrations,
+                                 const nsAString& aName,
+                                 EnabledState aEnabled)
+{
+  MOZ_ASSERT(IsCustomPropertyName(aName));
+  if (!aRegistrations ||
+      (!nsLayoutUtils::CSSVariablesEnabled() &&
+       aEnabled != EnabledState::eIgnoreEnabledState)) {
+    return mozilla::CSSProperty(eCSSProperty_UNKNOWN);
+  }
+
+  CSSVariableRegistration* registration;
+  if (!aRegistrations->mData.Get(Substring(aName,
+                                           CSS_CUSTOM_NAME_PREFIX_LENGTH),
+                                 &registration)) {
+    return mozilla::CSSProperty(eCSSProperty_UNKNOWN);
+  }
+
+  return mozilla::CSSProperty(registration->mAtom);
+}
+
 nsCSSFontDesc
 nsCSSProps::LookupFontDesc(const nsACString& aFontDesc)
 {
   MOZ_ASSERT(gFontDescTable, "no lookup table, needs addref");
   nsCSSFontDesc which = nsCSSFontDesc(gFontDescTable->Lookup(aFontDesc));
 
   if (which == eCSSFontDesc_Display &&
       !Preferences::GetBool("layout.css.font-display.enabled")) {

--- a/layout/style/nsCSSProps.h
+++ b/layout/style/nsCSSProps.h
@@ -15,16 +15,21 @@
 #include <type_traits>
 #include "nsString.h"
 #include "nsCSSPropertyID.h"
 #include "nsStyleStructFwd.h"
 #include "nsCSSKeywords.h"
 #include "mozilla/CSSEnabledState.h"
 #include "mozilla/UseCounter.h"
 
+namespace mozilla {
+struct CSSVariableRegistrations;
+class CSSProperty;
+} // namespace mozilla
+
 // Length of the "--" prefix on custom names (such as custom property names,
 // and, in the future, custom media query names).
 #define CSS_CUSTOM_NAME_PREFIX_LENGTH 2
 
 // Flags for ParseVariant method
 #define VARIANT_KEYWORD         0x000001  // K
 #define VARIANT_LENGTH          0x000002  // L
 #define VARIANT_PERCENT         0x000004  // P
@@ -392,16 +397,25 @@ public:
   // eCSSPropertyExtra_variable won't be returned from these methods.
   static nsCSSPropertyID LookupPropertyByIDLName(
       const nsAString& aPropertyIDLName,
       EnabledState aEnabled);
   static nsCSSPropertyID LookupPropertyByIDLName(
       const nsACString& aPropertyIDLName,
       EnabledState aEnabled);
 
+  // Look up a registered custom property as a CSSProperty by its full name
+  // (with the '--' prefix). If the given custom property has not been
+  // registered, or if custom properties are not enabled, we return
+  // CSSProperty(eCSSProperty_UNKNOWN).
+  static mozilla::CSSProperty
+  LookupCustomProperty(const mozilla::CSSVariableRegistrations* aRegistrations,
+                       const nsAString& aName,
+                       EnabledState aEnabled);
+
   // Returns whether aProperty is a custom property name, i.e. begins with
   // "--".  This assumes that the CSS Variables pref has been enabled.
   static bool IsCustomPropertyName(const nsAString& aProperty);
   static bool IsCustomPropertyName(const nsACString& aProperty);
 
   static inline bool IsShorthand(nsCSSPropertyID aProperty) {
     MOZ_ASSERT(0 <= aProperty && aProperty < eCSSProperty_COUNT,
                "out of range");