diff --git a/types/webidl2/index.d.ts b/types/webidl2/index.d.ts
index ff74059675..013bae8cf2 100644
--- a/types/webidl2/index.d.ts
+++ b/types/webidl2/index.d.ts
@@ -29,11 +29,7 @@ export type IDLInterfaceMemberType =
     | DeclarationMemberType
     | OperationMemberType;
 
-export type IDLInterfaceMixinMemberType =
-    | AttributeMemberType
-    | ConstantMemberType
-    | (DeclarationMemberType & { type: "stringifier" })
-    | OperationMemberType;
+export type IDLInterfaceMixinMemberType = AttributeMemberType | ConstantMemberType | OperationMemberType;
 
 export type IDLNamespaceMemberType = AttributeMemberType | OperationMemberType;
 
diff --git a/types/webidl2/webidl2-tests.ts b/types/webidl2/webidl2-tests.ts
index 75ce356c4f..b13be46ed5 100644
--- a/types/webidl2/webidl2-tests.ts
+++ b/types/webidl2/webidl2-tests.ts
@@ -13,30 +13,35 @@ for (const rootType of parsed) {
     switch (rootType.type) {
         case "interface mixin":
             rootType; // $ExpectType InterfaceMixinType
-            logMembers(rootType.members);
             console.log(rootType.partial);
+            logInterfaceMixinMembers(rootType.members);
             break;
+
         case "interface":
             rootType; // $ExpectType InterfaceType
             console.log(rootType.inheritance);
-            logMembers(rootType.members);
             console.log(rootType.partial);
+            logInterfaceMembers(rootType.members);
             break;
+
         case "namespace":
             rootType; // $ExpectType NamespaceType
             console.log(rootType.partial);
             logNamespaceMembers(rootType.members);
             break;
+
         case "callback interface":
             rootType; // $ExpectType CallbackInterfaceType
             console.log(rootType.inheritance);
-            logMembers(rootType.members);
             console.log(rootType.partial);
+            logCallbackInterfaceMembers(rootType.members);
             break;
+
         case "callback":
             rootType; // $ExpectType CallbackType
             logArguments(rootType.arguments);
             break;
+
         case "dictionary":
             rootType; // $ExpectType DictionaryType
             console.log(rootType.inheritance);
@@ -49,6 +54,7 @@ for (const rootType of parsed) {
                 }
             }
             break;
+
         case "enum":
             rootType; // $ExpectType EnumType
             for (const v of rootType.values) {
@@ -56,15 +62,18 @@ for (const rootType of parsed) {
                 console.log(v.value);
             }
             break;
+
         case "typedef":
             rootType; // $ExpectType TypedefType
             logIdlType(rootType.idlType);
             break;
+
         case "includes":
             rootType; // $ExpectType IncludesType
             console.log(rootType.target);
             console.log(rootType.includes);
             break;
+
         default:
             rootType; // $ExpectType never
             break;
@@ -74,73 +83,145 @@ for (const rootType of parsed) {
     logExtAttrs(rootType.extAttrs);
 }
 
-function logMembers(members: webidl2.IDLInterfaceMemberType[]) {
+function logInterfaceMembers(members: webidl2.IDLInterfaceMemberType[]) {
     for (const member of members) {
-        switch (member.type) {
-            case "constructor":
-                member; // $ExpectType ConstructorMemberType
-                member.parent; // $ExpectType InterfaceType
-                logArguments(member.arguments);
-                break;
-            case "operation":
-                member; // $ExpectType OperationMemberType
-                member.parent; // $ExpectType CallbackInterfaceType | InterfaceMixinType | InterfaceType | NamespaceType
-                logNamespaceMember(member);
-                break;
-            case "attribute":
-                member; // $ExpectType AttributeMemberType
-                member.parent; // $ExpectType InterfaceMixinType | InterfaceType | NamespaceType
-                logNamespaceMember(member);
-                break;
-            case "const":
-                member; // $ExpectType ConstantMemberType
-                member.parent; // $ExpectType CallbackInterfaceType | InterfaceMixinType | InterfaceType
-                console.log(member.name);
-                logIdlType(member.idlType);
-                logValueDescription(member.value);
-                console.log(member.nullable);
-                break;
-            case "iterable":
-                member; // $ExpectType IterableDeclarationMemberType
-                member.parent; // $ExpectType InterfaceMixinType | InterfaceType
-                member.async; // $ExpectType boolean
-                member.readonly; // $ExpectType false
-                member.idlType; // $ExpectType [IDLTypeDescription] | [IDLTypeDescription, IDLTypeDescription]
-                member.idlType.forEach(logIdlType);
-                member.arguments; // $ExpectType Argument[]
-                logArguments(member.arguments);
-                break;
-            case "maplike":
-                member; // $ExpectType MaplikeDeclarationMemberType
-                member.parent; // $ExpectType InterfaceMixinType | InterfaceType
-                member.async; // $ExpectType false
-                member.readonly; // $ExpectType boolean
-                member.idlType; // $ExpectType [IDLTypeDescription, IDLTypeDescription]
-                member.idlType.forEach(logIdlType);
-                member.arguments; // $ExpectType []
-                logArguments(member.arguments);
-                break;
-            case "setlike":
-                member; // $ExpectType SetlikeDeclarationMemberType
-                member.parent; // $ExpectType InterfaceMixinType | InterfaceType
-                member.async; // $ExpectType false
-                member.readonly; // $ExpectType boolean
-                member.idlType; // $ExpectType [IDLTypeDescription]
-                member.idlType.forEach(logIdlType);
-                member.arguments; // $ExpectType []
-                logArguments(member.arguments);
-                break;
-            default:
-                member; // $ExpectType never
-                break;
-        }
+        logInterfaceMember(member);
         logExtAttrs(member.extAttrs);
     }
 }
 
+function logInterfaceMember(member: webidl2.IDLInterfaceMemberType) {
+    switch (member.type) {
+        case "constructor":
+            member; // $ExpectType ConstructorMemberType
+            member.parent; // $ExpectType InterfaceType
+            logArguments(member.arguments);
+            break;
+
+        case "operation":
+            member; // $ExpectType OperationMemberType
+            member.parent; // $ExpectType CallbackInterfaceType | InterfaceMixinType | InterfaceType | NamespaceType
+            logInterfaceMixinMember(member);
+            break;
+
+        case "attribute":
+            member; // $ExpectType AttributeMemberType
+            member.parent; // $ExpectType InterfaceMixinType | InterfaceType | NamespaceType
+            logInterfaceMixinMember(member);
+            break;
+
+        case "const":
+            member; // $ExpectType ConstantMemberType
+            member.parent; // $ExpectType CallbackInterfaceType | InterfaceMixinType | InterfaceType
+            logInterfaceMixinMember(member);
+            break;
+
+        case "iterable":
+            member; // $ExpectType IterableDeclarationMemberType
+            member.parent; // $ExpectType InterfaceMixinType | InterfaceType
+            member.async; // $ExpectType boolean
+            member.readonly; // $ExpectType false
+            member.idlType; // $ExpectType [IDLTypeDescription] | [IDLTypeDescription, IDLTypeDescription]
+            member.idlType.forEach(logIdlType);
+            member.arguments; // $ExpectType Argument[]
+            logArguments(member.arguments);
+            break;
+
+        case "maplike":
+            member; // $ExpectType MaplikeDeclarationMemberType
+            member.parent; // $ExpectType InterfaceMixinType | InterfaceType
+            member.async; // $ExpectType false
+            member.readonly; // $ExpectType boolean
+            member.idlType; // $ExpectType [IDLTypeDescription, IDLTypeDescription]
+            member.idlType.forEach(logIdlType);
+            member.arguments; // $ExpectType []
+            logArguments(member.arguments);
+            break;
+
+        case "setlike":
+            member; // $ExpectType SetlikeDeclarationMemberType
+            member.parent; // $ExpectType InterfaceMixinType | InterfaceType
+            member.async; // $ExpectType false
+            member.readonly; // $ExpectType boolean
+            member.idlType; // $ExpectType [IDLTypeDescription]
+            member.idlType.forEach(logIdlType);
+            member.arguments; // $ExpectType []
+            logArguments(member.arguments);
+            break;
+
+        default:
+            member; // $ExpectType never
+            break;
+    }
+}
+
+function logInterfaceMixinMembers(members: webidl2.IDLInterfaceMixinMemberType[]) {
+    for (const member of members) {
+        logInterfaceMixinMember(member);
+        logExtAttrs(member.extAttrs);
+    }
+}
+
+function logInterfaceMixinMember(member: webidl2.IDLInterfaceMixinMemberType) {
+    switch (member.type) {
+        case "operation":
+            member; // $ExpectType OperationMemberType
+            member.parent; // $ExpectType CallbackInterfaceType | InterfaceMixinType | InterfaceType | NamespaceType
+            logCallbackInterfaceMember(member);
+            break;
+
+        case "attribute":
+            member; // $ExpectType AttributeMemberType
+            member.parent; // $ExpectType InterfaceMixinType | InterfaceType | NamespaceType
+            logNamespaceMember(member);
+            break;
+
+        case "const":
+            member; // $ExpectType ConstantMemberType
+            member.parent; // $ExpectType CallbackInterfaceType | InterfaceMixinType | InterfaceType
+            logCallbackInterfaceMember(member);
+            break;
+
+        default:
+            member; // $ExpectType never
+            break;
+    }
+}
+
+function logCallbackInterfaceMembers(members: webidl2.IDLCallbackInterfaceMemberType[]) {
+    for (const member of members) {
+        logCallbackInterfaceMember(member);
+        logExtAttrs(member.extAttrs);
+    }
+}
+
+function logCallbackInterfaceMember(member: webidl2.IDLCallbackInterfaceMemberType) {
+    switch (member.type) {
+        case "operation":
+            member; // $ExpectType OperationMemberType
+            member.parent; // $ExpectType CallbackInterfaceType | InterfaceMixinType | InterfaceType | NamespaceType
+            logNamespaceMember(member);
+            break;
+
+        case "const":
+            member; // $ExpectType ConstantMemberType
+            member.parent; // $ExpectType CallbackInterfaceType | InterfaceMixinType | InterfaceType
+            console.log(member.name);
+            logIdlType(member.idlType);
+            logValueDescription(member.value);
+            console.log(member.nullable);
+            break;
+
+        default:
+            member; // $ExpectType never
+            break;
+    }
+}
+
 function logNamespaceMembers(members: webidl2.IDLNamespaceMemberType[]) {
     for (const member of members) {
         logNamespaceMember(member);
+        logExtAttrs(member.extAttrs);
     }
 }
 
@@ -153,12 +234,14 @@ function logNamespaceMember(member: webidl2.IDLNamespaceMemberType) {
             console.log(member.name);
             console.log(member.special);
             break;
+
         case "attribute":
             member; // $ExpectType AttributeMemberType
             member.parent; // $ExpectType InterfaceMixinType | InterfaceType | NamespaceType
             console.log(member.name);
             console.log(member.special, member.readonly, member.inherit);
             break;
+
         default:
             member; // $ExpectType never
             break;
@@ -184,12 +267,14 @@ function logExtAttr(extAttr: webidl2.ExtendedAttribute) {
                 rhs.value; // $ExpectType string
                 logExtAttrRHS(rhs);
                 break;
+
             case "identifier-list":
             case "string-list":
             case "decimal-list":
             case "integer-list":
                 logExtAttrRHSList(rhs);
                 break;
+
             default:
                 rhs; // $ExpectType never
                 break;
@@ -245,6 +330,10 @@ function logIdlType(idlType: webidl2.IDLTypeDescription) {
             case "record":
                 idlType.idlType.length; // $ExpectType 2
                 break;
+
+            default:
+                idlType; // $ExpectType never
+                break;
         }
     } else {
         idlType; // $ExpectType SingleTypeDescription
@@ -262,36 +351,44 @@ function logValueDescription(valueDesc: webidl2.ValueDescription) {
             valueDesc.value; // $ExpectType string
             console.log(valueDesc.value);
             break;
+
         case "number":
             valueDesc; // $ExpectType ValueDescriptionNumber
             valueDesc.value; // $ExpectType string
             console.log(valueDesc.value);
             break;
+
         case "boolean":
             valueDesc; // $ExpectType ValueDescriptionBoolean
             valueDesc.value; // $ExpectType boolean
             console.log(valueDesc.value);
             break;
+
         case "null":
             valueDesc; // $ExpectType ValueDescriptionNull
             break;
+
         case "Infinity":
             valueDesc; // $ExpectType ValueDescriptionInfinity
             valueDesc.negative; // $ExpectType boolean
             console.log(valueDesc.negative);
             break;
+
         case "NaN":
             valueDesc; // $ExpectType ValueDescriptionNaN
             break;
+
         case "sequence":
             valueDesc; // $ExpectType ValueDescriptionSequence
             valueDesc.value; // $ExpectType []
             valueDesc.value.length; // $ExpectType 0
             console.log(valueDesc.value);
             break;
+
         case "dictionary":
             valueDesc; // $ExpectType ValueDescriptionDictionary
             break;
+
         default:
             valueDesc; // $ExpectType never
             break;