<svg style="border: 5px solid black" xmlns="http://www.w3.org/2000/svg" x="-100px" y="-100px" width="300px" height="300px">
<script type="text/javascript">
document.rootElement.x.baseVal.value = 0;
document.rootElement.y.baseVal.value = 0;
document.rootElement.width.baseVal.value = 100;
document.rootElement.height.baseVal.value = 100;
</script>
<rect x="-100" y="-100" width="300" height="300" fill="red"/>
<rect x="0" y="0" width="100" height="100" fill="green"/>
</svg>