Hi Jake_Carter and welcome to WebmasterWorld :)
As the other posters have said, divs aren't allowed inside lists. The recommendations have more about
Lists [w3.org]
You could make each of those divs a <li> and give those list items the class menu_right and menu_left, but an easier solution for the issue is to reduce the html elements rather than add more. From your code it looks like the divs are being used to create a "hook" on which you can "hang" an image to create a rounded corner on the bottom-left and bottom-right of the menu.
If you are only supporting modern browsers it is possible to use css3 for rounded corners or use multiple images and multiple backgrounds, or use them and accept older browsers will only see a menu with square bottom corners.
If you must support older browsers, another way is to set the bottom-left corner image on the ul.
Currently you have:
<ul class="menu">
<li></li>< --- redundant
<div class="menu_right"></div> <--not valid and can be avoided
<div class="menu_left"></div> <--not valid and can be avoided
<li class="active"><a href="/" title="Home">Home</a></li>
<li><a href="#" class="#" title="Login">Login</a></li>
<li><a href="#" class="#" title="Signup">Signup</a></li>
</ul>
In your css change
ul.menu {
background-color: #006699;
...}
to
ul.menu {
background: #006699 url("../img/menu_left.png") no-repeat left bottom;
...}
That avoids the need to have an element and class "menu_left" just to create a round corner on left bottom.
Then set menu_right.png on the right-most <li> to create the round corner on the right bottom of the menu. (You may have to adjust the widths of you <ul> and the <li>'s to get it to lay out in the correct visual place.)
That would reduce your html to
<ul class="menu">
<li class="active"><a href="/" title="Home">Home</a></li>
<li><a href="#" class="#" title="Login">Login</a></li>
<li><a href="#" class="#" title="Signup">Signup</a></li>
</ul>
which is much cleaner and also valid.
[edited by: alt131 at 11:33 pm (utc) on Apr 15, 2011]