diff --git a/layout/style/StyleRule.cpp b/layout/style/StyleRule.cpp
index 2f08d88f14..f4a3d05e5b 100644
--- a/layout/style/StyleRule.cpp
+++ b/layout/style/StyleRule.cpp
@@ -920,9 +920,13 @@ nsCSSSelector::AppendToStringWithoutCombinatorsOrNegations
     // has a ":" that can't be escaped and (b) all pseudo-classes at
     // this point are known, and therefore we know they don't need
     // escaping.
-    aString.Append(temp);
+    if (!nsCSSPseudoClasses::IsHiddenFromSerialization(list->mType)) {
+      aString.Append(temp);
+    }
     if (list->u.mMemory) {
-      aString.Append(char16_t('('));
+      if (!nsCSSPseudoClasses::IsHiddenFromSerialization(list->mType)) {
+        aString.Append(char16_t('('));
+      }
       if (nsCSSPseudoClasses::HasStringArg(list->mType)) {
         nsStyleUtil::AppendEscapedCSSIdent(
           nsDependentString(list->u.mString), aString);
@@ -951,7 +955,9 @@ nsCSSSelector::AppendToStringWithoutCombinatorsOrNegations
         list->u.mSelectorList->ToString(tmp, aSheet);
         aString.Append(tmp);
       }
-      aString.Append(char16_t(')'));
+      if (!nsCSSPseudoClasses::IsHiddenFromSerialization(list->mType)) {
+        aString.Append(char16_t(')'));
+      }
     }
   }
 }
diff --git a/layout/style/nsCSSParser.cpp b/layout/style/nsCSSParser.cpp
index 23620e35af..26d04a2d82 100644
--- a/layout/style/nsCSSParser.cpp
+++ b/layout/style/nsCSSParser.cpp
@@ -67,6 +67,7 @@ static bool sWebkitPrefixedAliasesEnabled;
 static bool sWebkitDevicePixelRatioEnabled;
 static bool sMozGradientsEnabled;
 static bool sControlCharVisibility;
+static bool sLegacyNegationPseudoClassEnabled;
 
 const uint32_t
 nsCSSProps::kParserVariantTable[eCSSProperty_COUNT_no_shorthands] = {
@@ -6144,7 +6145,6 @@ CSSParserImpl::ParsePseudoSelector(int32_t&       aDataMask,
 #ifdef MOZ_XUL
        isTree ||
 #endif
-       CSSPseudoClassType::negation == pseudoClassType ||
        nsCSSPseudoClasses::HasStringArg(pseudoClassType) ||
        nsCSSPseudoClasses::HasNthPairArg(pseudoClassType) ||
        nsCSSPseudoClasses::HasSelectorListArg(pseudoClassType)) &&
@@ -6182,25 +6182,26 @@ CSSParserImpl::ParsePseudoSelector(int32_t&       aDataMask,
     }
   }
 
-  if (!parsingPseudoElement &&
-      CSSPseudoClassType::negation == pseudoClassType) {
-    if (aIsNegated) { // :not() can't be itself negated
-      REPORT_UNEXPECTED_TOKEN(PEPseudoSelDoubleNot);
-      UngetToken();
-      return eSelectorParsingStatus_Error;
-    }
-    // CSS 3 Negation pseudo-class takes one simple selector as argument
-    nsSelectorParsingStatus parsingStatus =
-      ParseNegatedSimpleSelector(aDataMask, aSelector);
-    if (eSelectorParsingStatus_Continue != parsingStatus) {
-      return parsingStatus;
-    }
-  }
-  else if (!parsingPseudoElement && isPseudoClass) {
+  if (!parsingPseudoElement && isPseudoClass) {
     aDataMask |= SEL_MASK_PCLASS;
     if (eCSSToken_Function == mToken.mType) {
       nsSelectorParsingStatus parsingStatus;
-      if (nsCSSPseudoClasses::HasStringArg(pseudoClassType)) {
+      if (sLegacyNegationPseudoClassEnabled &&
+          CSSPseudoClassType::negation == pseudoClassType) {
+        // :not() can't be itself negated
+        if (aIsNegated) {
+          REPORT_UNEXPECTED_TOKEN(PEPseudoSelDoubleNot);
+          UngetToken();
+          return eSelectorParsingStatus_Error;
+        }
+        // CSS 3 Negation pseudo-class takes one simple selector as argument
+        parsingStatus =
+          ParseNegatedSimpleSelector(aDataMask, aSelector);
+        if (eSelectorParsingStatus_Continue != parsingStatus) {
+          return parsingStatus;
+        }
+      }
+      else if (nsCSSPseudoClasses::HasStringArg(pseudoClassType)) {
         parsingStatus =
           ParsePseudoClassWithIdentArg(aSelector, pseudoClassType);
       }
@@ -6583,8 +6584,6 @@ CSSParserImpl::ParsePseudoClassWithNthPairArg(nsCSSSelector& aSelector,
 
 //
 // Parse the argument of a pseudo-class that has a selector list argument.
-// Such selector lists cannot contain combinators, but can contain
-// anything that goes between a pair of combinators.
 //
 CSSParserImpl::nsSelectorParsingStatus
 CSSParserImpl::ParsePseudoClassWithSelectorListArg(nsCSSSelector& aSelector,
@@ -6612,22 +6611,35 @@ CSSParserImpl::ParsePseudoClassWithSelectorListArg(nsCSSSelector& aSelector,
     return eSelectorParsingStatus_Error; // our caller calls SkipUntil(')')
   }
 
-  for (nsCSSSelectorList *l = slist; l; l = l->mNext) {
-    nsCSSSelector *s = l->mSelectors;
-    if (s == nullptr) {
-      MOZ_ASSERT(isForgiving,
-                 "unexpected empty selector in unforgiving selector list");
-      break;
+  // Special handling for the :not() pseudo-class.
+  if (aType == CSSPseudoClassType::negation) {
+    nsCSSSelector* negations = &aSelector;
+    while (negations->mNegations) {
+      negations = negations->mNegations;
     }
-    // Check that none of the selectors in the list have combinators or
-    // pseudo-elements.
-    if ((!isForgiving && s->mNext) || s->IsPseudoElement()) {
-      return eSelectorParsingStatus_Error; // our caller calls SkipUntil(')')
+    // XXX: Use a special internal-only pseudo-class to handle selector lists.
+    // TODO: This should only happen if we don't have a simple selector.
+    nsCSSSelector* newSel = new nsCSSSelector();
+    newSel->AddPseudoClass(CSSPseudoClassType::mozAnyPrivate, slist.forget());
+    negations->mNegations = newSel;
+  } else {
+    for (nsCSSSelectorList *l = slist; l; l = l->mNext) {
+      nsCSSSelector *s = l->mSelectors;
+      if (s == nullptr) {
+        MOZ_ASSERT(isForgiving,
+                   "unexpected empty selector in unforgiving selector list");
+        break;
+      }
+      // Check that none of the selectors in the list have combinators or
+      // pseudo-elements.
+      if ((!isForgiving && s->mNext) || s->IsPseudoElement()) {
+        return eSelectorParsingStatus_Error; // our caller calls SkipUntil(')')
+      }
     }
-  }
 
-  // Add the pseudo with the selector list parameter
-  aSelector.AddPseudoClass(aType, slist.forget());
+    // Add the pseudo with the selector list parameter
+    aSelector.AddPseudoClass(aType, slist.forget());
+  }
 
   // close the parenthesis
   if (!ExpectSymbol(')', true)) {
@@ -17891,6 +17903,8 @@ nsCSSParser::Startup()
                                "layout.css.prefixes.gradients");
   Preferences::AddBoolVarCache(&sControlCharVisibility,
                                "layout.css.control-characters.visible");
+  Preferences::AddBoolVarCache(&sLegacyNegationPseudoClassEnabled,
+                               "layout.css.legacy-negation-pseudo.enabled");
 }
 
 nsCSSParser::nsCSSParser(mozilla::css::Loader* aLoader,
diff --git a/layout/style/nsCSSPseudoClassList.h b/layout/style/nsCSSPseudoClassList.h
index 740ebcc422..cbe3bd8f92 100644
--- a/layout/style/nsCSSPseudoClassList.h
+++ b/layout/style/nsCSSPseudoClassList.h
@@ -133,6 +133,11 @@ CSS_PSEUDO_CLASS(mozBrowserFrame, ":-moz-browser-frame",
 // matching operation.
 CSS_PSEUDO_CLASS(scope, ":scope", 0, "layout.css.scope-pseudo.enabled")
 
+// Matches selectors inside the selector list argument. Unlike :is(),
+// this is unforgiving and hidden from serialization.
+CSS_PSEUDO_CLASS(mozAnyPrivate, ":-moz-any-private",
+                 CSS_PSEUDO_CLASS_ENABLED_IN_UA_SHEETS_AND_CHROME, "")
+
 // :not needs to come at the end of the non-bit pseudo-class list, since
 // it doesn't actually get directly matched on in SelectorMatches.
 CSS_PSEUDO_CLASS(negation, ":not", 0, "")
diff --git a/layout/style/nsCSSPseudoClasses.cpp b/layout/style/nsCSSPseudoClasses.cpp
index 23517ef52c..0fc460a514 100644
--- a/layout/style/nsCSSPseudoClasses.cpp
+++ b/layout/style/nsCSSPseudoClasses.cpp
@@ -122,7 +122,9 @@ bool
 nsCSSPseudoClasses::HasSelectorListArg(Type aType)
 {
   return HasForgivingSelectorListArg(aType) ||
+         aType == Type::negation ||
          aType == Type::mozAny ||
+         aType == Type::mozAnyPrivate ||
          aType == Type::host ||
          aType == Type::hostContext;
 }
@@ -133,6 +135,12 @@ nsCSSPseudoClasses::HasOptionalSelectorListArg(Type aType)
   return aType == Type::host;
 }
 
+bool
+nsCSSPseudoClasses::IsHiddenFromSerialization(Type aType)
+{
+  return aType == Type::mozAnyPrivate;
+}
+
 void
 nsCSSPseudoClasses::PseudoTypeToString(Type aType, nsAString& aString)
 {
diff --git a/layout/style/nsCSSPseudoClasses.h b/layout/style/nsCSSPseudoClasses.h
index 99c05b019f..ff2da74ff0 100644
--- a/layout/style/nsCSSPseudoClasses.h
+++ b/layout/style/nsCSSPseudoClasses.h
@@ -62,6 +62,7 @@ public:
   static bool HasForgivingSelectorListArg(Type aType);
   static bool HasSelectorListArg(Type aType);
   static bool HasOptionalSelectorListArg(Type aType);
+  static bool IsHiddenFromSerialization(Type aType);
   static bool IsUserActionPseudoClass(Type aType);
 
   // Should only be used on types other than Count and NotPseudoClass
diff --git a/layout/style/nsCSSRuleProcessor.cpp b/layout/style/nsCSSRuleProcessor.cpp
index ebc3dad63e..692419b566 100644
--- a/layout/style/nsCSSRuleProcessor.cpp
+++ b/layout/style/nsCSSRuleProcessor.cpp
@@ -1952,6 +1952,17 @@ static bool SelectorMatches(Element* aElement,
         }
         break;
 
+      case CSSPseudoClassType::mozAnyPrivate:
+        {
+          if (!SelectorListMatches(aElement,
+                                   pseudoClass,
+                                   aNodeMatchContext,
+                                   aTreeMatchContext)) {
+            return false;
+          }
+        }
+        break;
+
       case CSSPseudoClassType::host:
         {
           // In order to match :host, the element must be a shadow root host,
diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index 9478c92bd8..10bd5aaf46 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -2551,6 +2551,9 @@ pref("layout.css.prefixes.webkit", true);
 // pref is set to false.)
 pref("layout.css.prefixes.device-pixel-ratio-webkit", false);
 
+// Is the legacy negation pseudo-class behavior enabled?
+pref("layout.css.legacy-negation-pseudo.enabled", false);
+
 // Is support for the :is() and :where() selectors enabled?
 pref("layout.css.is-where-pseudo.enabled", true);