index.html

<!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">
  <link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
<link href="https://fonts.googleapis.com/css2?family=Roboto:wght@300&display=swap" rel="stylesheet">
  <link rel="stylesheet" href="./styles.css">
  <script src="script.js" defer></script>
  <title>Age Calc</title>
</head>
<body>
  <div class="container">
    <h1 class="heading">How old am I if I was born on 
      <span><input type="date" id="input" class="date"></span>
    </h1>
    
    <button id="button" class="btn">Calculate</button>
    <div id="output" class="result"></div>
  </div>
</body>
</html>