<!DOCTYPE html>
<html>
<body>
<script>
function myFunction() { alert('Starting to load video');
}
</script>
<video controls onloadstart="myFunction()"> <source src="https://file-examples.com/wp-content/storage/2017/04/file_example_MP4_480_1_5MG.mp4" type="video/mp4"> <source src="https://file-examples.com/wp-content/storage/2017/04/file_example_MP4_480_1_5MG.mp4" type="video/ogg"> Your browser does not support HTML5 video.
</video>
<p>Video courtesy of <a href="https://www.bigbuckbunny.org/" target="_blank">Big Buck Bunny</a>.</p>
</body>
</html>