diff --git a/html/browsers/the-window-object/length-attribute.window.js b/html/browsers/the-window-object/length-attribute.window.js
new file mode 100644
index 00000000000000..d56e1e46928043
--- /dev/null
+++ b/html/browsers/the-window-object/length-attribute.window.js
@@ -0,0 +1,24 @@
+async_test(t => {
+  const frame = document.createElement("iframe");
+  frame.srcdoc = "<iframe name=x srcdoc='<iframe name=z></iframe>'></iframe><iframe name=y></iframe>";
+  frame.onload = t.step_func_done(() => {
+    const frameW = frame.contentWindow;
+    assert_equals(frameW.length, 2);
+    assert_not_equals(frameW.x, undefined);
+    assert_not_equals(frameW.y, undefined);
+    assert_equals(frameW.z, undefined);
+    assert_equals(frameW.x, frameW[0]);
+    assert_equals(frameW.y, frameW[1]);
+    const xFrameW = frameW.x;
+    assert_equals(xFrameW.length, 1);
+    assert_not_equals(xFrameW.z, undefined);
+    assert_equals(xFrameW.z, xFrameW[0]);
+    frame.remove();
+    assert_equals(frameW.length, 0);
+    assert_equals(frameW.x, undefined);
+    assert_equals(frameW[0], undefined);
+    assert_equals(xFrameW.length, 0);
+    assert_equals(xFrameW.z, undefined);
+  });
+  document.body.append(frame);
+}, "Window object's length IDL attribute (and named access)");