Preload CSS/JavaScript without execution

April 21st, 2010. Tagged: CSS, images, JavaScript, performance

Preloading components in advance is good for performance. There are several ways to do it. But even the cleanest solution (open up an iframe and go crazy there) comes at a price - the price of the iframe and the price of parsing and executing the preloaded CSS and JavaScript. There's also a relatively high risk of potential JavaScript errors if the script you preload assumes it's loaded in a page different than the one that preloads.

After a bit of trial and lot of error I think I came up with something that could work cross-browser:

  • in IE use new Image().src to preload all component types
  • in all other browsers use a dynamic <object> tag

Code and demo

Here's the final solution, below are some details.

In this example I assume the page prefetches after onload some components that will be needed by the next page. The components are a CSS, a JS and a PNG (sprite).

window.onload = function () {
    var i = 0,
        max = 0,
        o = null,
        // list of stuff to preload
        preload = [
            '<?php echo $id; ?>.sleep.expires.png',
            '<?php echo $id; ?>.sleep.expires.js',
            '<?php echo $id; ?>.sleep.expires.css'
        isIE = navigator.appName.indexOf('Microsoft') === 0;
    for (i = 0, max = preload.length; i < max; i += 1) {
        if (isIE) {
            new Image().src = preload[i];
        o = document.createElement('object'); = preload[i];
        // IE stuff, otherwise 0x0 is OK
        //o.width = 1;
        //o.height = 1;
        // = "hidden";
        //o.type = "text/plain"; // IE 
        o.width  = 0;
        o.height = 0;
        // only FF appends to the head
        // all others require body

A demo is here:
In the demo the components are delayed with 1 second each and sent with Expries header. Feel free to increment the ID for a new test with uncached components.

Tested in FF3.6, O10, Safari 4, Chrome 5, IE 6,7,8.


  • new Image().src doesn't do the job in FF because it has a separate cache for images. Didn't seem to work in Safari either where CSS and JS were requested on the second page where they sould've been cached
  • the dynamic object element has to be outside the head in most browsers in order to fire off the downloads
  • dynamic object works also in IE7,8 with a few tweaks (commented out in the code above) but not in IE6. In a separate tests I've also found the object element to be expensive in IE in general.

That's about it. Below are some unsuccessful attempts I tried which failed for various reasons in different browsers.

Other unsuccessful attempts

I was actually inspired by this post by Ben Cherry where he loads CSS and JS in a print stylesheet. Clever hack, unfortunately didn't work in Chrome which caches the JS but doesn't execute it on the next page.

One of the comments on Ben's post suggested (Philip and Dejan said the same) using invalid type attribute to prevent execution, e.g. text/cache.

var s = document.createElement('script');
s.src = preload[1];
s.type = "text/cache";

That worked for the most parts but not in FF3.6 where the JavaScript was never requested.

A dynamic link prefetch didn't do anything, not even in FF which is probably the only browser that supports this.

for (i = 0, max = preload.length; i < max; i += 1) {
    var link = document.createElement('link');
    link.href = preload[i];
    link.rel = "prefetch";

Then it took a bit of trial/error to make IE7,8 work with an object tag, before I stumbled into IE6 and gave up in favor of image src.

In conclusion

I believe this is a solution I could be comfortable with, although it involves user agent sniffing. It certainly looks less hacky than loading JS as CSS anyways. And object elements are meant to load any type of component so no semantic conflict here I don't believe. Feel free to test and report any edge cases or browser/OS combos. (JS errors in IE on the second page are ok, because I'm using console.log in the preloaded javascript)

Thanks for reading!

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