I have the following script that could be use in pre-HTML5 and a current HTML5 program.

Is there a way to use the pre-HTML version if the current browser does not support HTML5?

How do you determine if HTML5 is the working browser so that I would know which version of the <input> tag to use?

Note: I don't know if this is purely a JS problem or just an HTML set-up problem.

Code:
<!DOCTYPE HTML>
<html>
<head>
<title> Untitled </title>
<meta charset="utf-8">
<script type="text/javascript">

// Cross-browser compliant "PlaceHolder" function
// From: http://help.dottoro.com/ljgugboo.php

function ClearPlaceHolder (input) { if (input.value == input.defaultValue) { input.value = ""; } }
function SetPlaceHolder (input) { if (input.value == "") { input.value = input.defaultValue; } }

</script>
</head> 
<body>
 <input type="text" value="Please fill this field" onfocus="ClearPlaceHolder(this)" onblur="SetPlaceHolder(this)" /> 

 <input type="text" value="" placeholder="Zipcode" />
</body>

</body>
</html>