--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -1448,16 +1448,17 @@ nsIDocument::nsIDocument()
 
 // NOTE! nsDocument::operator new() zeroes out all members, so don't
 // bother initializing members to 0.
 
 nsDocument::nsDocument(const char* aContentType)
   : nsIDocument()
   , mAnimatingImages(true)
   , mViewportType(Unknown)
+  , mCSSVariableRegistrations(new mozilla::CSSVariableRegistrations)
 {
   SetContentTypeInternal(nsDependentCString(aContentType));
 
   if (gDocumentLeakPRLog)
     MOZ_LOG(gDocumentLeakPRLog, LogLevel::Debug,
            ("DOCUMENT %p created", this));
 
   // Start out mLastStyleSheetSet as null, per spec
@@ -1508,16 +1509,22 @@ nsDocument::IsAboutPage()
   principal->GetURI(getter_AddRefs(uri));
   bool isAboutScheme = true;
   if (uri) {
     uri->SchemeIs("about", &isAboutScheme);
   }
   return isAboutScheme;
 }
 
+mozilla::CSSVariableRegistrations*
+nsDocument::GetCSSVariableRegistrations() const
+{
+  return mCSSVariableRegistrations;
+}
+
 nsDocument::~nsDocument()
 {
   if (gDocumentLeakPRLog)
     MOZ_LOG(gDocumentLeakPRLog, LogLevel::Debug,
            ("DOCUMENT %p destroyed", this));
 
   NS_ASSERTION(!mIsShowing, "Destroying a currently-showing document");
 

--- a/dom/base/nsDocument.h
+++ b/dom/base/nsDocument.h
@@ -64,16 +64,17 @@
 #include "nsDataHashtable.h"
 #include "mozilla/TimeStamp.h"
 #include "mozilla/Attributes.h"
 #include "nsIDOMXPathEvaluator.h"
 #include "jsfriendapi.h"
 #include "ImportManager.h"
 #include "mozilla/LinkedList.h"
 #include "CustomElementsRegistry.h"
+#include "mozilla/CSSVariableRegistrations.h"
 
 #define XML_DECLARATION_BITS_DECLARATION_EXISTS   (1 << 0)
 #define XML_DECLARATION_BITS_ENCODING_EXISTS      (1 << 1)
 #define XML_DECLARATION_BITS_STANDALONE_EXISTS    (1 << 2)
 #define XML_DECLARATION_BITS_STANDALONE_YES       (1 << 3)
 
 
 class nsDOMStyleSheetSetList;
@@ -1793,16 +1794,23 @@ private:
   // Set to true when the document is possibly controlled by the ServiceWorker.
   // Used to prevent multiple requests to ServiceWorkerManager.
   bool mMaybeServiceWorkerControlled;
 
 #ifdef DEBUG
 public:
   bool mWillReparent;
 #endif
+
+public:
+  mozilla::CSSVariableRegistrations*
+  GetCSSVariableRegistrations() const override;
+
+private:
+  RefPtr<mozilla::CSSVariableRegistrations> mCSSVariableRegistrations;
 };
 
 class nsDocumentOnStack
 {
 public:
   explicit nsDocumentOnStack(nsDocument* aDoc) : mDoc(aDoc)
   {
     mDoc->IncreaseStackRefCnt();

--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -86,16 +86,17 @@ class nsTextNode;
 class nsWindowSizes;
 class nsDOMCaretPosition;
 class nsViewportInfo;
 class nsIGlobalObject;
 struct nsCSSSelectorList;
 
 namespace mozilla {
 class CSSStyleSheet;
+struct CSSVariableRegistrations;
 class ErrorResult;
 class EventStates;
 class PendingAnimationTracker;
 class StyleSetHandle;
 class SVGAttrAnimationRuleProcessor;
 template<typename> class OwningNonNull;
 
 namespace css {
@@ -3214,16 +3215,20 @@ protected:
   // Flags for use counters used by any child documents of this document.
   std::bitset<mozilla::eUseCounter_Count> mChildDocumentUseCounters;
   // Flags for whether we've notified our top-level "page" of a use counter
   // for this child document.
   std::bitset<mozilla::eUseCounter_Count> mNotifiedPageForUseCounter;
 
   // Whether the user has interacted with the document or not:
   bool mUserHasInteracted;
+
+public:
+  virtual mozilla::CSSVariableRegistrations*
+  GetCSSVariableRegistrations() const = 0;
 };
 
 NS_DEFINE_STATIC_IID_ACCESSOR(nsIDocument, NS_IDOCUMENT_IID)
 
 /**
  * mozAutoSubtreeModified batches DOM mutations so that a DOMSubtreeModified
  * event is dispatched, if necessary, when the outermost mozAutoSubtreeModified
  * object is deleted.

new file mode 100644
--- /dev/null
+++ b/layout/style/CSSVariableRegistration.h
@@ -0,0 +1,103 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* data about a registered CSS Custom property */
+
+#ifndef mozilla_CSSVariableRegistration_h
+#define mozilla_CSSVariableRegistration_h
+
+#include "mozilla/CSSValueType.h"
+#include "mozilla/CSSVariableSyntax.h"
+#include "nsCOMPtr.h"
+#include "nsCSSScanner.h"
+#include "nsCSSValue.h"
+#include "nsStringFwd.h"
+
+class nsIAtom;
+class nsStyleContext;
+
+namespace mozilla {
+
+// CSSVariableExprContext specifies the context of the expression for a
+// custom property (either as the 'initialValue' of a PropertyDescriptor passed
+// to CSS.registerProperty or in a '--property: value' declaration).
+// It is necessary for properties that might involve resource requests or
+// relative URIs (in particular, URL values for <url>s or <image>s.)
+// The members names have the same meanings as they do in nsCSSValue and
+// nsCSSValue).
+struct CSSVariableExprContext
+{
+  CSSVariableExprContext()
+    : mSheetURI(nullptr)
+    , mBaseURI(nullptr)
+    , mSheetPrincipal(nullptr) { }
+
+  CSSVariableExprContext(nsIURI* aSheetURI, nsIURI* aBaseURI,
+                         nsIPrincipal* aSheetPrincipal)
+    : mSheetURI(aSheetURI)
+    , mBaseURI(aBaseURI)
+    , mSheetPrincipal(aSheetPrincipal) { }
+
+  nsCOMPtr<nsIURI> mSheetURI;
+  nsCOMPtr<nsIURI> mBaseURI;
+  nsCOMPtr<nsIPrincipal> mSheetPrincipal;
+};
+
+// CSSVariableRegistration holds registration information for a particular
+// custom property.
+// It is populated based on the values provided to the CSS.registerProperty
+// call exposed as part of the Properties & Values API.
+struct CSSVariableRegistration
+{
+  bool mInherited;
+  CSSVariableSyntax mSyntax;
+
+  // mInitialExpr is the exact string the user specified as as 'initialValue' in
+  // the PropertyDescriptor dictionary.
+  // It is always a non-empty string, unless the user did not specify any
+  // 'initialValue' at all:
+  // If mInitialExpr is non-empty, it is guaranteed to parse to a valid and
+  // computationally idempotent CSS value with mSyntax.
+  //
+  // "If no initialValue is provided and the syntax is *, then a special initial
+  //  value used [sic]. This initial value must be considered parseable by
+  //  registerProperty() but invalid at computed value time."
+  //
+  // See: https://drafts.css-houdini.org/css-properties-values-api/#the-registerproperty-function
+  // See also:
+  // - CSSVariableResolver::VisitNode
+  // - nsCSSParser::ResolveValueWithVariableReferencesRec
+  nsString mInitialExpr;
+
+  // mInitialValue and mInitialType are the value and type of the expression in
+  // mInitialExpr if mInitialExpr is supplied (if mInitialExpr is empty, that
+  // means no 'initialValue' was supplied, and the values of these members are
+  // defined).
+  // Which unit & type to parse to are determined by mSyntax.
+  nsCSSValue mInitialValue;
+  CSSValueType mInitialType;
+
+  // mInitialContext ends up being used when we construct token stream values
+  // (in CSSVariableResolver for unregistered variables and in
+  // nsCSSParser::ParseTypedValue for values with syntax type <anything>) and
+  // when nsCSSParser calls SetValueToURL (and possibly other places in
+  // nsCSSParser that use mSheetURI, etc.)
+  CSSVariableExprContext mInitialContext;
+
+  // mInitialFirstToken and mInitialLastToken end up being used inside of
+  // CSSParserImpl::ResolveVariableValues. It calls AppendTokens to construct
+  // the resulting expanded value, and needs to know whether to insert /**/
+  // between adjacent tokens.
+  nsCSSTokenSerializationType mInitialFirstToken;
+  nsCSSTokenSerializationType mInitialLastToken;
+
+  // mAtom is the variable name, without the '--' prefix.
+  nsCOMPtr<nsIAtom> mAtom;
+};
+
+} // namespace mozilla
+
+#endif // mozilla_CSSVariableRegistration_h

new file mode 100644
--- /dev/null
+++ b/layout/style/CSSVariableRegistrations.cpp
@@ -0,0 +1,30 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/CSSVariableRegistrations.h"
+
+#include "mozilla/CSSVariableRegistration.h"
+#include "nsGlobalWindow.h"
+#include "nsIDocument.h"
+#include "nsStyleContext.h"
+
+namespace mozilla {
+
+CSSVariableRegistrations*
+CSSVariableRegistrationsOfDocument(const nsIDocument* aDoc)
+{
+  return aDoc ? aDoc->GetCSSVariableRegistrations()
+              : nullptr;
+}
+
+CSSVariableRegistrations*
+CSSVariableRegistrationsOfStyleContext(const nsStyleContext* aStyleContext)
+{
+  nsIDocument* doc = aStyleContext->PresContext()->Document();
+  return CSSVariableRegistrationsOfDocument(doc);
+}
+
+} // namespace mozilla

new file mode 100644
--- /dev/null
+++ b/layout/style/CSSVariableRegistrations.h
@@ -0,0 +1,62 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* A mapping from custom property names to registration information. */
+
+#ifndef mozilla_CSSVariableRegistrations_h
+#define mozilla_CSSVariableRegistrations_h
+
+#include <stdint.h>
+
+#include "mozilla/CSSVariableRegistration.h"
+#include "nsClassHashtable.h"
+#include "nsISupportsImpl.h"
+
+class nsIDocument;
+class nsStyleContext;
+
+namespace mozilla {
+
+/**
+ * CSSVariableRegistrations, a mapping from registered custom property names to
+ * their registrations. Whenever a registration in |mData| is modified,
+ * |mGeneration| *must* be incremented. (This happens in CSS.registerProperty
+ * and CSS.unregisterProperty). The reason we need to update |mGeneration| is
+ * because a Declaration stores a reference to a CSSVariableRegistration that it
+ * uses to determine which declaration of a given property is valid.
+ *
+ * Unlike standard properties, for which we can decide validity at parse time
+ * (unless they contain variable references), registered custom property types
+ * are not necessarily known at parse time. In particular, CSS.registerProperty,
+ * which registers the type of a custom property, can be called in the middle of
+ * the life of a Declaration. This means we have to recompute validities every
+ * time we try to read from a Declaration and the types of custom properties
+ * have changed. We store this monotonically increasing version number so that
+ * we only recompute when necessary.
+ */
+struct CSSVariableRegistrations
+{
+  typedef nsClassHashtable<nsStringHashKey, CSSVariableRegistration> Data;
+
+  NS_INLINE_DECL_REFCOUNTING(CSSVariableRegistrations);
+
+  uint32_t mGeneration = 0;
+  Data mData;
+
+private:
+  ~CSSVariableRegistrations() = default;
+};
+
+CSSVariableRegistrations*
+CSSVariableRegistrationsOfDocument(const nsIDocument* aDoc);
+
+CSSVariableRegistrations*
+CSSVariableRegistrationsOfStyleContext(
+    const nsStyleContext* aStyleContext);
+
+} // namespace mozilla
+
+#endif // mozilla_CSSVariableRegistrations_h

--- a/layout/style/moz.build
+++ b/layout/style/moz.build
@@ -83,16 +83,18 @@ EXPORTS += [
 EXPORTS.mozilla += [
     'AnimationCollection.h',
     'CSSComputedValue.h',
     'CSSEnabledState.h',
     'CSSProperty.h',
     'CSSStyleSheet.h',
     'CSSValueType.h',
     'CSSVariableDeclarations.h',
+    'CSSVariableRegistration.h',
+    'CSSVariableRegistrations.h',
     'CSSVariableResolver.h',
     'CSSVariableSyntax.h',
     'CSSVariableValues.h',
     'HandleRefPtr.h',
     'IncrementalClearCOMRuleArray.h',
     'LayerAnimationInfo.h',
     'RuleNodeCacheConditions.h',
     'RuleProcessorCache.h',
@@ -144,16 +146,17 @@ UNIFIED_SOURCES += [
     'AnimationCommon.cpp',
     'CounterStyleManager.cpp',
     'CSS.cpp',
     'CSSComputedValue.cpp',
     'CSSLexer.cpp',
     'CSSRuleList.cpp',
     'CSSStyleSheet.cpp',
     'CSSVariableDeclarations.cpp',
+    'CSSVariableRegistrations.cpp',
     'CSSVariableResolver.cpp',
     'CSSVariableSyntax.cpp',
     'CSSVariableValues.cpp',
     'Declaration.cpp',
     'ErrorReporter.cpp',
     'FontFace.cpp',
     'FontFaceSet.cpp',
     'FontFaceSetIterator.cpp',