index.html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <link rel="stylesheet" href="style.css">
  <title>Birthday</title>
</head>
<body>
  <section>
    <h1 class="shadow">Happy birthday</h1>
    <div class="banner">
      <div class="blocks"></div>
    </div>
  </section>


  <script>
    // const banner=document.querySelector('.banner');
    // const blocks=document.querySelector('.blocks');
    const banner=document.getElementsByClassName('banner')[0];
    const blocks=document.getElementsByClassName('blocks');
    for(var i=1;i<400;i++){
      banner.innerHTML+="<div class='blocks'></div>";
      blocks[i].style.animationDelay=`${i*0.05}s`
    }
  </script>
</body>
</html>