In the first line <! DOCTYPE html> it is given to the browser to indicate what kind of document it is.

<!DOCTYPE html>
 <html>
    <head>
      <title> 3schools</title>
    </head>
    <body>
      <h1>This is heading 1</h1>
      <h2>This is heading 2</h2>
      <h3>This is heading 3</h3>
      <h4>This is heading 4</h4>
      <h5>This is heading 5</h5>
     </body>
  </html>



This declaration is given based on what version of HTML uses.A document of HTML 5 such as the above document.


  Anyway, it is not very important.
If you want you can give it. But if you do not give it, then there is not a proper output in some browsers. Such Explorer.


Declaration is given in HTML 5.

<!DOCTYPE html⁢>