<svg xmlns="http://www.w3.org/2000/svg">
<defs>
    <clipPath id="clip">
        <rect width="100" height="100" opacity=".4"/>
        <rect/> <!-- trigger render-to-image-buffer mode -->
    </clipPath>
</defs>

<!-- Should result in a 100x100 green square -->
<rect width="200" height="200" fill="red" clip-path="url(#clip)"/>
<rect width="100" height="100" fill="green"/>

</svg>