Sometimes it's the little things that matter most. I was experimenting with XHTML for this site, and managed to generate a page that didn't display properly at all. I was using both IE and Phoenix at the time, and they (impressively) rendered the garbage exactly the same. I selected IE's View Source option to try to understand what went wrong. I couldn't see what was wrong. Using Phoenix's View Page Source, it became clear, because while IE launches Notepad to view source, Phoenix uses its own syntax-colored window.

An empty <script> tag (because it references a remote JavaScript file) was rendered like this in XHTML:

<script type='text/javascript' src='main.js' />

Where HTML had:

<script type='text/javascript' src='main.js'></script>

Both browsers treated everything between the opening script tag and the next explicit closing script tag as script. As a result, the XHTML version had the first two-thirds of the page swallowed (incorrectly) as script. The syntax coloring made this clear.

I assume there's a proper setting of DTDs to get XHTML to accept a truly empty script tag. In any case, Phoenix saved me a bunch of time finding the reason for the wreckage of the page.

BTW: Phoenix has been renamed Firebird, and there are tons of good reasons to use it.