Tryit Editor v1.8

Source Code:
<!DOCTYPE html> 
<html> 
<body> 

<button onclick="enablePreload()" type="button">Enable preload</button>
<button onclick="disablePreload()" type="button">Disable preload</button>
<button onclick="checkPreload()" type="button">Check preload status</button>
<br> 
<video id="video1" controls="controls">
  <source src="mov_bbb.mp4" type="video/mp4">
  <source src="mov_bbb.ogg" type="video/ogg">
  Your browser does not support HTML5 video.
</video>

<script>
myVid=document.getElementById("video1");
function enablePreload()
  { 
  myVid.preload="auto";
  } 
function disablePreload()
  { 
  myVid.preload="none";
  } 
function checkPreload()
  { 
  alert(myVid.preload);
  } 
</script> 

<p>Video courtesy of <a href="http://www.bigbuckbunny.org/" target="_blank">Big Buck Bunny</a>.</p>

</body> 
</html>
Result:
W3Schools.com - Try it yourself