<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div class="parent">
    <div class="child"></div>
    <div class="child"></div>
    <div class="child"></div>
  </div>
</body>
</html>
.parent {
  background-color:skyblue;
  width:500px;
/*   padding: 10px; */
/*   margin-bottom:20px; */
  
}

.child {
  margin-top:10px;
  background-color:yellow;
  width:400px;
  height:100px;
}

External CSS

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External JavaScript

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