<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<defs>
    <filter id="filter0">
        <feGaussianBlur stdDeviation="4"/>
    </filter>

    <filter id="filter1" filter="url(#filter1)">
        <feGaussianBlur stdDeviation="4"/>
    </filter>

    <!-- WebKit currently does not implement filter chaining -->
    <filter id="filter2" xlink:href="#filter2"/>
</defs>

<!-- This should be blurred -->
<rect x="50" y="0" width="100" height="100" filter="url(#filter1)" fill="blue"/>

<!-- This should not render anything -->
<rect x="50" y="150" width="100" height="100" filter="url(#filter2)" fill="blue"/>

<line x1="175" x2="175" y1="0" y2="250" stroke="red"/>
<text text-anchor="middle" x="175" y="300">Both sides of the red line should look identical</text>

<!-- This is the reference rendering -->
<g transform="translate(150,0)">
    <rect x="50" y="0" width="100" height="100" filter="url(#filter0)" fill="blue"/>
</g>
</svg>