diff --git a/cjs/dom/token-list.js b/cjs/dom/token-list.js
index 25f70f1d..b4999a7e 100644
--- a/cjs/dom/token-list.js
+++ b/cjs/dom/token-list.js
@@ -67,13 +67,16 @@ class DOMTokenList extends Set {
    * @param {boolean?} force
    */
   toggle(token, force) {
+    // Web IDL optional boolean: omitted or undefined → plain toggle
+    if (arguments.length < 2 || force === undefined)
+      force = !this.has(token);
     if (this.has(token)) {
       if (force)
         return true;
       this.delete(token);
       update(this);
     }
-    else if (force || arguments.length === 1) {
+    else if (force) {
       super.add(token);
       update(this);
       return true;
diff --git a/esm/dom/token-list.js b/esm/dom/token-list.js
index 27c19ff7..e5b1c8ed 100644
--- a/esm/dom/token-list.js
+++ b/esm/dom/token-list.js
@@ -66,13 +66,16 @@ export class DOMTokenList extends Set {
    * @param {boolean?} force
    */
   toggle(token, force) {
+    // Web IDL optional boolean: omitted or undefined → plain toggle
+    if (arguments.length < 2 || force === undefined)
+      force = !this.has(token);
     if (this.has(token)) {
       if (force)
         return true;
       this.delete(token);
       update(this);
     }
-    else if (force || arguments.length === 1) {
+    else if (force) {
       super.add(token);
       update(this);
       return true;
diff --git a/test/html/element.js b/test/html/element.js
index 3c3fedd8..561ed282 100644
--- a/test/html/element.js
+++ b/test/html/element.js
@@ -82,6 +82,17 @@ assert(node.classList.value, 'a c', 'correct .value again');
 assert(node.classList.replace('b', 'c'), false, 'replace did not happen');
 assert(node.classList.supports('whatever'), true, 'whatever');
 
+// #277: toggle(token, undefined) must match omitted force (plain toggle)
+const tokenList = document.createElement('div');
+assert(tokenList.classList.toggle('foo', undefined), true, 'undefined force adds missing token');
+assert(tokenList.className, 'foo', 'undefined force added class');
+assert(tokenList.classList.toggle('foo', undefined), false, 'undefined force removes present token');
+assert(tokenList.className, '', 'undefined force removed class');
+tokenList.classList.toggle('bar', true);
+assert(tokenList.classList.contains('bar'), true, 'force true adds');
+tokenList.classList.toggle('bar', false);
+assert(tokenList.classList.contains('bar'), false, 'force false removes');
+
 assert(node.onclick, null, 'Level 0 events');
 
 let args = null;