Важность использования тега в HTML-документах 

<!DOCTYPE html>
<html>
<head>
</head>
<body>
  <h1>Document Content</h1>
  <!-- Document content goes here -->
</body>
</html>
<!DOCTYPE html>
<html>
<head>
  <script>
    document.addEventListener('DOMContentLoaded', function() {
      document.title = 'Dynamic Title';
    });
  </script>
</head>
<body>
  <h1>Document Content</h1>
  <!-- Document content goes here -->
</body>
</html>
<!DOCTYPE html>
<html>
<head>
  <?php
    $pageTitle = 'Server-Side Title';
    echo "<title>$pageTitle</title>";
  ?>
</head>
<body>
  <h1>Document Content</h1>
  <!-- Document content goes here -->
</body>
</html>