The web site we're developing is JavaScript based, so we need to warn users if they come to our site with JavaScript disabled. The easiest way to do this is to use the HTML <noscript> tag (http://www.w3schools.com/TAGS/tag_noscript.asp).
For example:
<head>
<script ...
</script>
</head>
<body>
<script ...></script>
<noscript>
<div class="noscript-warning">
<h1>Please turn on JavaScript to use this site.</h1>
<p>
It seems that you have JavaScript turned off. You will need
to turn it on in order to use this site.
</p>
</div>
</noscript>
</body>
The contents of the <noscript> tag will only be displayed when JavaScript is disabled. When JavaScript is enabled, the tag and its contents will not be displayed at all.
