Stretch horizontal ul to fit width of div

People hate on tables for non-tabular data, but what you're asking for is exactly what tables are good at. <table width="100%">

inelegant (but effective) way: use percentages

#horizontal-style {
    width: 100%;
}

li {
    width: 20%;
}

This only works with the 5 <li> example. For more or less, modify your percentage accordingly. If you have other <li>s on your page, you can always assign these particular ones a class of "menu-li" so that only they are affected.

This is the easiest way to do it: http://jsfiddle.net/thirtydot/jwJBd/

(or with table-layout: fixed for even width distribution: http://jsfiddle.net/thirtydot/jwJBd/59/)

This won't work in IE7.

#horizontal-style {
    display: table;
    width: 100%;
    /*table-layout: fixed;*/
}
#horizontal-style li {
    display: table-cell;
}
#horizontal-style a {
    display: block;
    border: 1px solid red;
    text-align: center;
    margin: 0 5px;
    background: #999;
}

_{Old answer before your edit: http://jsfiddle.net/thirtydot/DsqWr/}