remove unneeded type="text/css" from demo (#106)

and update polyfill.io to v3.

demos/network-idle.html

@@ -6,8 +6,8 @@
     <meta http-equiv="X-UA-Compatible" content="IE=edge">
     <title>network-idle-callback demo</title>
     <meta name="viewport" content="width=device-width, initial-scale=1">
-    <link rel="stylesheet" type="text/css" media="screen" href="../test/main.css" />
-    <script src="https://polyfill.io/v2/polyfill.min.js?features=IntersectionObserver"></script>
+    <link rel="stylesheet" media="screen" href="../test/main.css" />
+    <script src="https://polyfill.io/v3/polyfill.min.js?features=IntersectionObserver"></script>
     <script>
         if ('serviceWorker' in navigator) {
             window.addEventListener('load', function () {
@@ -37,4 +37,4 @@
     </script>
 </body>
 
-</html>
+</html>