<!DOCTYPE html>
<html>
    <body>
        <p>
        <button onclick="test()">Test</button>
        </p>

        <script>

            let _element;
            function elementType()
            {
                return _element.toString();
            }
            
            function test(elementType)
            {
                _element = document.createElement(elementType);
                _element.onerror = (event) => {
                    if (window.webkit)
                      window.webkit.messageHandlers.testHandler.postMessage('error');
                }
                _element.oncanplay = (event) => {
                    if (window.webkit)
                      window.webkit.messageHandlers.testHandler.postMessage('canplay');
                }
                _element.controls = "true"
                _element.src = "test.mp4";

                document.body.appendChild(_element);
            }

        </script>
    </body>
</html>