Check out example codes for "doctype html". It will help you in understanding the concepts better.

Code Example 1

<!doctype html>
<html lang="fr">
<head>
  <meta charset="utf-8">
  <title>Titre de la page</title>
  <link rel="stylesheet" href="style.css">
  <script src="script.js"></script>
</head>
<body>
  ...
  <!-- Le reste du contenu -->
  ...
</body>
</html>

Code Example 2

<!DOCTYPE html>
This tag shows that it is running HTML5

Code Example 3

<!-- HTML 5 DOCTYPE -->
<!DOCTYPE html>

Code Example 4

<!DOCTYPE html>
<!--Use the html tag too, under the declaration. -->

Code Example 5

<!DOCKTYPE html> <!-- Start of html code -->
<html>
  <head>
    <title>TITLE</title>
    <script>
      //JavaScript
    </script>
    <style>
      /* CSS */
    </style>
  </head>
  <body>
    <p>Hello!</p>
  </body>
</html> <!-- And another Thing I had to fix, sorry for not adding all the slashes, thats why it did not work for some.-->

Code Example 6

<!DOCTYPE html>
it lets the browser know about the version of HTML you
are using.but it is not nessesary 
since your browser will render it anyways

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