There is some code of angular 2 code. It can't pass w3c validation because of the tag . Or angular library somehow changes the source in the end to make it valid?
<!DOCTYPE html>
<html>
<head>
<title>Angular 2 QuickStart</title>
...
</head>
<!-- 3. Display the application -->
<body>
<my-app>Loading...</my-app>
</body>
</html>