<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg viewBox="0 0 400 300" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<defs>
    <g id="g">
        <g id="g2">
            <rect id="rect" stroke-width="5px" stroke="navy" width="60" height="10"/>
        </g>
        <circle id="circle" opacity="0.5" fill="green" cx="30" cy="5" r="10"/>
    </g>
</defs>

<use x="25" y="25" xlink:href="#g"/>
<use x="125" y="25" xlink:href="#g"/>

<text font-size="10" id="text" x="10" y="70">You should see a red rectangle and a green circle.</text>

<script>
document.getElementById("g2").setAttribute("fill", "red");
</script>
</svg>