WebPageTest

p83822 · (This post was last modified: 04-12-2012 04:17 AM by p83822.)

I thought I'd have another look at this, now that dns-prefetch is official HTML5. I ran the following in FF (because I know how to disable FF's dns cache.) (And yes, this is truly vile!)

My expectation was that the look ahead preparser would recognise nodejs.org and http://www.w3.org as being in the html, but would not recognise http://www.w3schools.com as it is in the JS. My understanding being that look ahead preparsers do not read JS.

To my surprise, the DNS lookup for http://www.w3.org occurs after the sleep. (VRTA graphs this beautifully - see attached.) Pic1 is with dns-prefetch tags commented out, Pic2 with them in.

The horrid code?. Enjoy:

<html>
<head>
<meta charset="utf-8">

</head>
<body>

<img src='http://nodejs.org/images/logo.png'/>

<script type="">
(function() {
var startTime = new Date().getTime();
while (new Date().getTime() < startTime + 1000);
}())
</script>

<img src='http://www.w3.org/2008/site/images/twitter-bird'/>

<script type="text/javascript">
(function() {
var d = function() {
var img = document.createElement("img");
img.src = "http://www.w3schools.com/images/w3schoolslogo.gif";
var s0 = document.getElementsByTagName('script')[0];
s0.parentNode.insertBefore(img, s0);
};
setTimeout(d, 1000);
}())
</script>
</body>
</html>

Daftness. Nothing will happen during the sleep. D'oh! Nice to know that the prefetch directive now though.