Switch between screen and mobile view with javascript

header.html

<script>
if( /Android|webOS|iPhone|iPad|iPod|BlackBerry/i.test(navigator.userAgent) ) {
    var cssUrl = "media/css/handheld.css";

    var head = document.getElementsByTagName("head")[0];

    var link = document.createElement("link");

    link.rel = "stylesheet";
    link.type = "text/css";
    link.href = cssUrl;

    document.head.appendChild(link);
} else {
    var cssUrl = "media/css/style.css";

    var head = document.getElementsByTagName("head")[0];

    var link = document.createElement("link");

    link.rel = "stylesheet";
    link.type = "text/css";
    link.href = cssUrl;

    document.head.appendChild(link);
}
</script>