index.html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <link rel="stylesheet" href="styles.css">
</head>
<body>
  <h1 id="greeting">Hello!</h1>
  <input id="number" type="number" placeholder="pick a number">
  <button onclick="double()">multiply by 2</button>
  <span id="result"></span>

  <button onclick="makeBlue()">make shapes blue</button>
  <button onclick="makeSquare()">make all shapes square</button>
  <div class="shape circle"></div>
  <div class="shape circle"></div>
  <div class="shape circle"></div>
  <div class="shape circle"></div>

  <script src="index.js"></script>
</body>
</html>