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chromium/third_party/blink/web_tests/svg/foreignObject/mask.html 

<html>
<body style="margin: 0;">
<svg xmlns="http://www.w3.org/2000/svg">
<!-- The FO should be masked, with only the green half visible. -->
<defs>
<mask id="mask">
    <rect width="200" height="50" style="fill: #fff"/>
</mask>
</defs>
<foreignObject width="200" height="100" mask="url(#mask)" opacity=".5">
    <html xmlns="http://www.w3.org/1999/xhtml">
    <body>
        <div style="background: green; height: 50px;"></div>
        <div style="background: red; height: 50px;"></div>
    </body>
    </html>
</foreignObject>
</svg>
</body>
</html>