diff --git a/layout/style/nsCSSParser.cpp b/layout/style/nsCSSParser.cpp
index 45d78f93dc..ba97bbd3d7 100644
--- a/layout/style/nsCSSParser.cpp
+++ b/layout/style/nsCSSParser.cpp
@@ -6552,6 +6552,11 @@ CSSParserImpl::ParsePseudoClassWithSelectorListArg(nsCSSSelector& aSelector,
     return eSelectorParsingStatus_Error; // our caller calls SkipUntil(')')
   }
 
+  if (nsCSSPseudoClasses::HasSingleSelectorArg(aType) &&
+      slist->mNext) {
+    return eSelectorParsingStatus_Error; // our caller calls SkipUntil(')')
+  }
+
   for (nsCSSSelectorList *l = slist; l; l = l->mNext) {
     nsCSSSelector *s = l->mSelectors;
     if (s == nullptr) {
diff --git a/layout/style/nsCSSPseudoClasses.cpp b/layout/style/nsCSSPseudoClasses.cpp
index a174525b1c..928326e399 100644
--- a/layout/style/nsCSSPseudoClasses.cpp
+++ b/layout/style/nsCSSPseudoClasses.cpp
@@ -102,6 +102,13 @@ nsCSSPseudoClasses::HasNthPairArg(Type aType)
          aType == Type::nthLastOfType;
 }
 
+bool
+nsCSSPseudoClasses::HasSingleSelectorArg(Type aType)
+{
+  return aType == Type::host ||
+         aType == Type::hostContext;
+}
+
 void
 nsCSSPseudoClasses::PseudoTypeToString(Type aType, nsAString& aString)
 {
diff --git a/layout/style/nsCSSPseudoClasses.h b/layout/style/nsCSSPseudoClasses.h
index 4a4bbe188c..76fcef3f78 100644
--- a/layout/style/nsCSSPseudoClasses.h
+++ b/layout/style/nsCSSPseudoClasses.h
@@ -58,6 +58,7 @@ public:
   static Type GetPseudoType(nsIAtom* aAtom, EnabledState aEnabledState);
   static bool HasStringArg(Type aType);
   static bool HasNthPairArg(Type aType);
+  static bool HasSingleSelectorArg(Type aType);
   static bool HasForgivingSelectorListArg(Type aType) {
     return aType == Type::is ||
      aType == Type::matches ||