There are numerous homepages which utilize js to play pranks; there are numerous threads which detail ways to incorporate js into your profile here (or to link to an online but offsite server). I think search terms such as "nodelet" + "js" might be helpful.
And if that's a false trail, What are Nodelets? may offer some clues.
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Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
<code> <a> <b> <big>
<blockquote> <br /> <dd>
<dl> <dt> <em> <font>
<h1> <h2> <h3> <h4>
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<li> <nbsp> <ol> <p>
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<sub> <sup> <table>
<td> <th> <tr> <tt>
Snippets of code should be wrapped in
<code> tags not
<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
Want more info? How to link or
or How to display code and escape characters
are good places to start.