<!DOCTYPE html>
<html>
<body>

<video id="video1" controls="controls">
  <source src="/example/html5/mov_bbb.mp4" type="video/mp4">
  <source src="/example/html5/mov_bbb.ogg" type="video/ogg">
  Your browser does not support HTML5 video.
</video>

<script>
myVid=document.getElementById("video1");
myVid.ondurationchange=alert("The video duration has changed");
</script>

</body>
</html>