Why do the float elements not align side by side when using widths?

http://stackoverflow.com/questions/11005954/why-do-the-float-elements-not-align-side-by-side-when-using-widths

by Wexcode

HTML

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTMl 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
    <meta http-equiv="content-type" content="text/html; charset=UTF-8" />
    <meta http-equiv="content-language" content="en-us" />
    <meta name="keywords" content="" />
    <meta name="description" content="" />
    <meta name="author" content="" />
    <meta name="copyright" content="&copy; 2012" />

    <title>DIV example</title>

    <base href="" />

    <link rel="stylesheet" href="" />

    <style type="text/css">

    * {
        margin: 0px;
        padding: 0px;
        font-family: Arial, Verdana, sans-serif;
        font-size: 100%;
    }

    #content {
        width: 700px;
        margin-top: 20px;
        margin-right: auto;
        margin-bottom: 0px;
        margin-left: auto;
    }

    #leftnav {
        float: left;
        width: 200px;
        border-width: 1px;
        border-color: #000000;
        border-style: solid;
    }

    #rightnav {
        width: 200px;
        border-width: 1px;
        border-color: #000000;
        border-style: solid;
        overflow: hidden;
    }


    </style>
</head>
<body>
    <div id="container">
        <div id="content">
            <div id="leftnav">left nav</div>
            <div id="rightnav">right nav</div>
        </div>
    </div>
</body>
</html>