diff --git a/baselines/dom.generated.d.ts b/baselines/dom.generated.d.ts
index 5f1d24151..22161016b 100644
--- a/baselines/dom.generated.d.ts
+++ b/baselines/dom.generated.d.ts
@@ -6096,10 +6096,10 @@ interface GlobalEventHandlers {
      */
     ontimeupdate: ((this: GlobalEventHandlers, ev: Event) => any) | null;
     ontoggle: ((this: GlobalEventHandlers, ev: Event) => any) | null;
-    ontouchcancel: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
-    ontouchend: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
-    ontouchmove: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
-    ontouchstart: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
+    ontouchcancel?: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
+    ontouchend?: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
+    ontouchmove?: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
+    ontouchstart?: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
     ontransitioncancel: ((this: GlobalEventHandlers, ev: TransitionEvent) => any) | null;
     ontransitionend: ((this: GlobalEventHandlers, ev: TransitionEvent) => any) | null;
     ontransitionrun: ((this: GlobalEventHandlers, ev: TransitionEvent) => any) | null;
@@ -19886,10 +19886,10 @@ declare var onsuspend: ((this: Window, ev: Event) => any) | null;
  */
 declare var ontimeupdate: ((this: Window, ev: Event) => any) | null;
 declare var ontoggle: ((this: Window, ev: Event) => any) | null;
-declare var ontouchcancel: ((this: Window, ev: TouchEvent) => any) | null;
-declare var ontouchend: ((this: Window, ev: TouchEvent) => any) | null;
-declare var ontouchmove: ((this: Window, ev: TouchEvent) => any) | null;
-declare var ontouchstart: ((this: Window, ev: TouchEvent) => any) | null;
+declare var ontouchcancel: ((this: Window, ev: TouchEvent) => any) | null | undefined;
+declare var ontouchend: ((this: Window, ev: TouchEvent) => any) | null | undefined;
+declare var ontouchmove: ((this: Window, ev: TouchEvent) => any) | null | undefined;
+declare var ontouchstart: ((this: Window, ev: TouchEvent) => any) | null | undefined;
 declare var ontransitioncancel: ((this: Window, ev: TransitionEvent) => any) | null;
 declare var ontransitionend: ((this: Window, ev: TransitionEvent) => any) | null;
 declare var ontransitionrun: ((this: Window, ev: TransitionEvent) => any) | null;
diff --git a/inputfiles/overridingTypes.json b/inputfiles/overridingTypes.json
index 1084ac750..deee4b5fd 100644
--- a/inputfiles/overridingTypes.json
+++ b/inputfiles/overridingTypes.json
@@ -129,6 +129,18 @@
                     "property": {
                         "onerror": {
                             "override-type": "OnErrorEventHandler"
+                        },
+                        "ontouchcancel": {
+                            "required": 0
+                        },
+                        "ontouchend": {
+                            "required": 0
+                        },
+                        "ontouchmove": {
+                            "required": 0
+                        },
+                        "ontouchstart": {
+                            "required": 0
                         }
                     }
                 }
diff --git a/src/emitter.ts b/src/emitter.ts
index 2c129ad2c..0e4fdddee 100644
--- a/src/emitter.ts
+++ b/src/emitter.ts
@@ -617,8 +617,12 @@ export function emitWebIdl(webidl: Browser.WebIdl, flavor: Flavor) {
             else {
                 pType = convertDomTypeToTsType(p);
             }
-            const requiredModifier = p.required === undefined || p.required === 1 ? "" : "?";
+            const required = p.required === undefined || p.required === 1;
+            const requiredModifier = required || prefix ? "" : "?";
             pType = p.nullable ? makeNullable(pType) : pType;
+            if (!required && prefix) {
+                pType += " | undefined"
+            }
             const readOnlyModifier = p["read-only"] === 1 && prefix === "" ? "readonly " : "";
             printer.printLine(`${prefix}${readOnlyModifier}${p.name}${requiredModifier}: ${pType};`);
         }