<!DOCTYPE html>
<html lang="ru" class="main">
<head>
<meta charset="UTF-8">
<title>Page Transition</title>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/normalize/5.0.0/normalize.min.css">
</head>
<body>
<div> //Забыли добавить
<a class="header" href="https://google.com"><h1>Click Me</h1></a></div>
<div class="cover enter"/> //Забыли закрыть
<script src='https://cdnjs.cloudflare.com/ajax/libs/zepto/1.1.6/zepto.min.js'></script>
</body>
</html>