Here the script is not executed on loading but is triggered by the action attribute of a <form>:

Here's the code:

<form action="javascript:" method="post" name="form1"><br />

<input type="button" name="button1" value="Click me" 
    onclick="document.write('Hello world'); document.close();" />

</form>

The document.close(); is needed by Netscape to flush everything it has in memory to the screen before the execution of the script stops.