Yes, fixing the errors will get rid of the problem. Peculiar it's IE that chokes on it.
This is a very broken list, or rather several.
HTML
<div id="mainmenu">
<ul>
<ul style="text-decoration: underline; color: rgb(51, 51, 255);">
<li><a class="current" href="index.html">HOme</a></li>
</ul>
<li style="text-decoration: underline; color: rgb(51, 51, 255);"><a href="green.html">software</a></li>
<li style="text-decoration: underline; color: rgb(51, 51, 255);"><a href="orange.html">videos</a></li>
<ul style="text-decoration: underline; color: rgb(51, 51, 255);">
<li><a href="purple.html">cameras</a></li>
</ul>
<li style="text-decoration: underline; color: rgb(51, 51, 255);"><a href="red.html">websites</a></li>
<ul>
<ul>
<li style="text-decoration: underline; color: rgb(51, 51, 255);"><a href="black.html">training</a></li>
</ul>
</ul>
</ul>
</div>
You can't nest lists the way you do and you can't have LIs on the loose between lists. There is no need for nesting. Nor is there any need for more than one list. The DIV around it all isn't needed either, if you don't have any special plans for it. You could as w ell put the id on the UL. The
one UL that will be left when you have cleaned it up.
There is no reason to float the DIV that I can see. Same goes for the UL in case you remove the DIV.
The result should be one list with floated LIs. Less Is More.