<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">

    <script>
    function boom()
    {
        document.getElementById("u").setAttribute("width", "30");
        document.getElementById("p").remove();
    }
    window.addEventListener("load", boom);
    </script>

    <symbol id="p" viewBox="0 0 100 20"/>

    <use id="u" xlink:href="#p"/>

</svg>
