Tryit Editor v1.8

Source Code:

<!DOCTYPE html>
<html>
<head>
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.1/jquery.min.js">
</script>
<script>
$(document).ready(function(){
  $("#btn1").click(function(){
    $("div").width(500);
  });
  $("#btn2").click(function(){
    $("div").width("10em");
  });
  $("#btn3").click(function(){
    $("div").width("300pt");
  });
});
</script>
</head>
<body>

<button id="btn1">Set width of div to 500px</button>
<button id="btn2">Set width of div to 10em</button>
<button id="btn3">Set width of div to 300pt</button>
<p><b>Note:</b> Use "" for em, pt, etc.</p>

<div style="height:100px;width:300px;border:1px solid blue;background-color:lightblue;"></div><br>

</body>
</html>

Result:

W3Schools.com - Try it yourself

Edit the code above and click "Submit Code" to see the result.