HTML validation
Open W3C HTML validator.
Copy and paste following HTML code to the validator.
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>I'm very cool title for a cool page</title>
</head>
<body>
<section>
<button>A</button>
<button>B</button>
<button>C</button>
<button>D</button>
<button hidden>E</button>
</section>
<section hidden>
<h1>Hello H1!</h1>
<h2>Hello H2!</h2>
<h3>Hello H3!</h3>
</section>
<comment>
<!--I'm a comment-->
</comment>
<dialog-block>
<dialog open>always open</dialog>
</dialog-block>
</body>
</html>
Answer questions below based on the validator feedback.