<h1> contains a number because it's only one of six heading tags. These heading tags go from <h1> to <h6>.

Add the heading tags in ascending order to see what happens.

<html>
 <body>
  <h1>My favorite things</h1>
  <h2>Raindrops on roses</h2>
  <h3>Whiskers on kittens</h3>
 Bright copper kettles
 </body>
</html>
]]>

The ascending order is <h1>, <h2>, and finally <h3>.