<html>
<head></head>
<style 
    contenteditable="true"
    style="
    z-index:1;
    position:fixed;
    top:0;
    right:0;
    width:300px;
    background:#fff;
    min-height:100vh;
    display:block !important;
    overflow: auto;
    padding:14px;
    border-left: solid 1px #cccccc">

    body {min-height: 100vh;}

</style>

<body>
  <p>You can edit the CSS code on the right corner</p>
</body>
</html>

External CSS

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External JavaScript

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