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chromium/third_party/blink/web_tests/fast/forms/range/slider-thumb-stylability.html 

<html>
<head>
    <style>
        input::-webkit-slider-thumb {
            -webkit-appearance: none;
            background-color: green;
            width: 20px;
            height: 20px;
        }
    </style>
</head>
<body>
<!--
  This tests styling of the slider thumb.
  You should see a green 20x20px square.
-->
<input type="range" value="0" style="-webkit-appearance: none;">
</body>
</html>