Untitled

<head>
    <link rel="stylesheet" href="base.css" />
    <link rel="stylesheet" href="mobile.css" media="max-width: 320px" />

    <!-- OR -->

    <style type="text/css">
        /* base styles go here */

        @media (max-width: 320px)
        {
            /* mobile styles go here */
        }
    </style>
</head>