Stylish Mobile Frame

frame.css

/* Use the Stylish Firefox addon to get this working. Replace domain with your domain. */

@namespace url(http://www.w3.org/1999/xhtml);

@-moz-document domain("localhost") {
    body:before {
        width: 320px;
        height: 480px;
        top: 0;
        position: fixed;
        left: -moz-calc(50% - 160px);
    }
    body {
        width: 320px;
        height: 480px;
        position: relative;
        left: -moz-calc(50% - 160px);
    }
    html:before, html:after {
        content: "";
        display: block;
        position: fixed;
        height: 100%;
        width: -moz-calc(50% - 160px);
        background: black;
        top: 0;
        left: 0;
    }
    html:after {
        right: 0;
        left: auto;
    }
}