<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1 Basic//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11-basic.dtd">  
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"  id="svg-root" width="100%" height="100%" viewBox="0 0 800 600" onload="createMask()">

<rect x="0" y="100" width="800" height="100" fill="blue" mask="url(#dynMask)"/>
<g id="content"/>

<script>
    var content = document.getElementById("content");

    function createMask()
    {
        var mask = document.createElementNS("http://www.w3.org/2000/svg", "mask");
        mask.setAttribute("id", "dynMask");
        mask.setAttribute("maskUnits", "userSpaceOnUse");
        mask.setAttribute("x", "0");
        mask.setAttribute("y", "0");
        mask.setAttribute("width", "800");
        mask.setAttribute("height", "300");

        var rect = document.createElementNS("http://www.w3.org/2000/svg", "rect");
        rect.setAttribute("width", "800");
        rect.setAttribute("height", "300");
        rect.setAttribute("fill", "red");

        mask.appendChild(rect);
        content.appendChild(mask);
    }
</script>

</svg>