JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html PUBLIC  "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
<head>
<title>[TITLE]</title>
</head>
<body>
<!--- style --->
<style type="text/css">
body {
  margin: 0;
  padding: 0;
}

#cont {
  margin: 0 auto;
  margin-top: 10px;
  width: 640px;
}
#top {
  width: 640px;
  height: 30px;
  background: blue;
  position: fixed;
}
#post {
  background: grey;
} 
</style>
<!--- style end --->
<div id="cont">
  <div id="top"></div>
  <div id="post">
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  </div>
</div>
</div>
</body>
</html>