Доктип
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset=UTF-8>
<title>Hello World!</title>
</head>
<body>
</body>
</html>
Relieved Raccoon
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset=UTF-8>
<title>Hello World!</title>
</head>
<body>
</body>
</html><!DOCTYPE html>
This tag shows that it is running HTML5