Make sure the 'prototype' property on generated Web IDL interfaces is { DontDelete | ReadOnly }. On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE". PASS tryToDeletePrototype(window.HTMLElement) is false PASS tryToSetPrototype(window.HTMLElement, null) is false PASS tryToSetPrototype(window.HTMLElement, undefined) is false PASS tryToSetPrototype(window.HTMLElement, 1) is false PASS tryToSetPrototype(window.HTMLElement, window.Object.prototype) is false PASS successfullyParsed is true TEST COMPLETE