ns="http://www.w3.org/2000/svg">
    <!-- Default fill opacity: 1 -->
    <circle cx="50" cy="50" r="40" fill-opacity="1" />

    <circle cx="50" cy="50" r="40" />

    <!-- Fill opacity as a number -->
    <circle cx="150" cy="50" r="40"
            fill-opacity="0.7" />

    <!-- Fill opacity as a percentage -->
    <circle cx="250" cy="50" r="40"
            fill-opacity="50%" />

    <!-- Fill opacity as a CSS property -->
    <circle cx="350" cy="50" r="40"
            style="fill-opacity: .25;" />
</s