<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1 Tiny//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11-tiny.dtd">
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">

<!-- an1: Change opacity from 0 to 0.5 in 4s, a script at 2s will set the opacity CSS property to 1, fill is freeze so this won't have any visible effect, nor any effect to the computed style -->
<rect opacity="0" width="100" height="100" fill="green">
    <animate id="an1" attributeType="CSS" attributeName="opacity" fill="freeze" from="0" to="0.5" begin="0s" dur="4s"/>
</rect>

</svg>