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chromium/third_party/blink/web_tests/fast/css/calc-comments-allowed.html 

<!DOCTYPE html>
<html>
<head>
    <title>Comments inside calc() allowed</title>
    <style>
        div { height: 50px; background-color: green; font-size: 50px; }
        #t1 { width: calc(50px + 1em) }
        #t2 { width: calc(50px + /* Comment allowed */ 1em) }
    </style>
</head>
<body>
    <p>You should see a green square below</p>
    <div id="t1"></div>
    <div id="t2"></div>
</body>
</html>