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chromium/third_party/blink/web_tests/fast/frames/content-opacity-1.html 

<div style="float: left; background-image: url(../replaced/resources/1x1-green.png)">
    <iframe src="data:text/html,
        <body style='background-color: rgba(0, 0, 0, 0.5)'>
        </body>
    "></iframe>
</div>