="#smiley" />
    </svg>

    <!-- none -->
    <rect x="0" y="30" width="160" height="60">
        <title>none</title>
    </rect>
    <svg viewBox="0 0 100 100" width="160" height="60"
         preserveAspectRatio="none" x="0" y="30">
        <use href="#smiley" />
    </svg>
</svg>

<svg viewBox="0 0 400 100" xmlns="http://www.w3.org/2000/svg">
    <!-- Default fill opacity: 1 -->
    <circle cx="50" cy="50" r="40" fill-opacity="1" />

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