Test the constructor of WebKitMutationObserver On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE". PASS window.WebKitMutationObserver is non-null. PASS typeof WebKitMutationObserver.prototype.observe is "function" PASS typeof WebKitMutationObserver.prototype.disconnect is "function" PASS typeof observer.observe is "function" PASS typeof observer.disconnect is "function" PASS new MutationObserver({ handleEvent: function() {} }) threw exception TypeError: Callback argument must be a function. PASS new MutationObserver({}) threw exception TypeError: Callback argument must be a function. PASS new MutationObserver(42) threw exception TypeError: Callback argument must be a function. PASS new MutationObserver("foo") threw exception TypeError: Callback argument must be a function. PASS successfullyParsed is true TEST COMPLETE