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chromium/third_party/blink/web_tests/fast/css-generated-content/html-pseudo-background-color.html 

<!DOCTYPE html>

<p>Bug 104855: Block level pseudo elements bleed background color to &lt;html&gt;</p>

<p>Neither frame should be red.</p>

<iframe srcdoc='
    <style>
        html:before { 
            content: "";
            display: block;
            background: red;
        }
    </style>
'></iframe>

<iframe srcdoc='
    <style>
        html:after { 
            content: "";
            display: block;
            background: red;
        }
    </style>
'></iframe>