What is incorrect in the following code for a main heading?
<h1>Main Heading<h1>
<h1> should be inside a <header> tag
<h1>
<header>
<h1>Main Heading<h1> should be <h1>Main Heading</h1>
<h1>Main Heading</h1>
Main Heading should be inside a <span>
Main Heading
<span>
<h1> should be <h2>
<h2>