JSHTML provides a simple way of introducing "JavaScript-only" markup into your HTML documents. Think of it as the opposite of the NOSCRIPT tag.
<!-- JSHTML {{
<p class="info">This paragraph only appears when JavaScript is enabled. If you view...</p>
}} -->
You can customize the flags used to signify a JSHTML block. This, for example, is possible:
<p>Normal content, nothing going on here... <!--| <strong>Boo!!!</strong> |--> </p>
More information can be found here: http://james.padolsey.com/javascript/introducing-jshtml/