SSI #exec example - directory listing

This is what the source for this page looks like: 
<H1>Directory listing using the <CODE>#exec</CODE> SSI directive</H1>

<PRE>
<!--#exec cmd="ls -al"-->
</PRE>

<HR>

<H3> Here I forgot to use the HTML <CODE><PRE></CODE> tag:</H3>

<!--#exec cmd="ls -al"-->

Directory listing using the #exec SSI directive

[an error occurred while processing this directive]

Here I forgot to use the HTML <PRE> tag:

[an error occurred while processing this directive]