diff --git a/crates/obscura-js/js/bootstrap.js b/crates/obscura-js/js/bootstrap.js
index 4cd6739..46637cc 100644
--- a/crates/obscura-js/js/bootstrap.js
+++ b/crates/obscura-js/js/bootstrap.js
@@ -5094,7 +5094,7 @@ globalThis.__obscura_setInputFiles = function(el, specs) {
   try { el.dispatchEvent(globalThis.__obscura_markTrusted(new Event("change", { bubbles: true }))); } catch (_e) {}
 };
 globalThis.Event = class Event {
-  constructor(t,o={}) { this.type=t;this.bubbles=!!o.bubbles;this.cancelable=!!o.cancelable;this.composed=!!o.composed;this.defaultPrevented=false;this.target=null;this.currentTarget=null;this.eventPhase=0;this.timeStamp=Date.now();this._propagationStopped=false;this._immediatePropagationStopped=false; }
+  constructor(t,o={}) { if (arguments.length < 1) throw new TypeError("Failed to construct 'Event': 1 argument required, but only 0 present."); this.type=String(t);this.bubbles=!!o.bubbles;this.cancelable=!!o.cancelable;this.composed=!!o.composed;this.defaultPrevented=false;this.target=null;this.currentTarget=null;this.eventPhase=0;this.timeStamp=Date.now();this._propagationStopped=false;this._immediatePropagationStopped=false; }
   get isTrusted() { return _trustedEvents.has(this); }
   preventDefault() { if (this.cancelable) this.defaultPrevented=true; } stopPropagation(){ this._propagationStopped=true; } stopImmediatePropagation(){ this._propagationStopped=true; this._immediatePropagationStopped=true; }
   initEvent(type,bubbles,cancelable) { if (arguments.length < 1) throw new TypeError("Failed to execute 'initEvent' on 'Event': 1 argument required, but only 0 present."); this.type=String(type);this.bubbles=!!bubbles;this.cancelable=!!cancelable;this.defaultPrevented=false;this._propagationStopped=false;this._immediatePropagationStopped=false; }
@@ -5109,7 +5109,7 @@ globalThis.Event = class Event {
 };
 _markNative(Event);
 globalThis.CustomEvent = class extends Event {
-  constructor(t,o={}) { super(t,o);this.detail=o.detail; }
+  constructor(t,o={}) { if (arguments.length < 1) throw new TypeError("Failed to construct 'CustomEvent': 1 argument required, but only 0 present."); super(t,o);this.detail=o.detail!==undefined?o.detail:null; }
   // Legacy DOM Level 2 init; some libraries (Starbucks China bundle, older
   // analytics shims) still call createEvent('CustomEvent') + initCustomEvent
   // instead of new CustomEvent(...). See issue #41.
diff --git a/crates/obscura-js/src/runtime.rs b/crates/obscura-js/src/runtime.rs
index d2ded4d..7800d30 100644
--- a/crates/obscura-js/src/runtime.rs
+++ b/crates/obscura-js/src/runtime.rs
@@ -3545,6 +3545,34 @@ mod tests {
         assert_eq!(kind, serde_json::json!(true));
     }
 
+    #[test]
+    fn event_constructor_matches_webidl_conformance() {
+        // new Event()/new CustomEvent() must throw (type is a required arg),
+        // the type argument must be coerced to a string, CustomEvent.detail must
+        // default to null (not undefined), createEvent must still build a
+        // type-"" event, and an explicit detail must be preserved.
+        let mut rt = setup_runtime("<html><body></body></html>");
+        let v = rt
+            .evaluate(
+                "(function(){\
+                 var out=[];\
+                 try{new Event();out.push('no-throw')}catch(e){out.push(e.name)}\
+                 try{new CustomEvent();out.push('no-throw')}catch(e){out.push(e.name)}\
+                 out.push(new Event(123).type+':'+typeof new Event(123).type);\
+                 out.push(String(new CustomEvent('x').detail));\
+                 out.push(String(new CustomEvent('x',{detail:7}).detail));\
+                 out.push(new Event('click').type);\
+                 out.push(JSON.stringify(document.createEvent('Event').type));\
+                 return out.join('|');\
+                 })()",
+            )
+            .unwrap();
+        assert_eq!(
+            v,
+            serde_json::json!("TypeError|TypeError|123:string|null|7|click|\"\"")
+        );
+    }
+
     #[test]
     fn test_promise_rejection_event_requires_promise() {
         let mut rt = setup_runtime("<html><body></body></html>");