Create an html file with the contents:
<script /> Can’t see me
and load it in Firefox.
Notice anything? Well, if you are like me you will notice that nothing shows up. No error, the tag isn’t ignored and there is no user notification as to why it isn’t showing up. The reason is that the parser sees this as an opening tag and doesn’t find a closing tag by the end of the document. Because nothing is visible from within <script%gt; blocks, nothing is displayed. I didn’t even know this issue existed!
Reference:
http://hixie.ch/advocacy/xhtml