The doctype tells the web browser which version of HTML a webpage uses. Without a doctype, the web browser has to guess.

<html>
 <head>
  <title>My webpage</title>
</head>
<body>
  <h1>My favorite things</h1>
  <p>
   Raindrops on <em>roses</em><br>
   Whiskers on <strong>kittens</strong> 
  </p>
 </body>
</html>]]>