How to determine if CSS has been loaded?

After doing some research and writing up my answer, I stumbled upon this link that explains everything you need to know about CSS, when it is loaded and how you can check for it.

The link provided explains it so well, in fact, that I’m adding some quotes from it for future reference.
If you’re curious, my answer was going to be #2 and a variation of #4.

When is a stylesheet really loaded?

With that out of the way, let’s see what we have here.

// my callback function 
// which relies on CSS being loaded function
CSSDone() {
    alert('zOMG, CSS is done');

// load me some stylesheet 
var url = "",
    head = document.getElementsByTagName('head')[0],
    link = document.createElement('link');

link.type = "text/css"; 
link.rel = "stylesheet";
link.href = url;

// call CSSDone() when CSS arrives

Options for the magic part, sorted from nice-and-easy to ridiculous

  1. listen to link.onload
  2. listen to link.addEventListener(‘load’)
  3. listen to link.onreadystatechange
  4. setTimeout and check for changes in document.styleSheets
  5. setTimeout and check for changes in the styling of a specific element you create but style with the new CSS

5th option is too crazy and assumes you have control over the content of the CSS, so forget it. Plus it checks for current styles in a timeout meaning it will flush the reflow queue and can be potentially slow. The slower the CSS to arrive, the more reflows. So, really, forget it.

So how about implementing the magic?


// #1   
link.onload = function () {
    CSSDone('onload listener');

// #2   
if (link.addEventListener) {
    link.addEventListener('load', function() {
        CSSDone("DOM's load event");
    }, false);   

// #3   
link.onreadystatechange = function() {
    var state = link.readyState;
    if (state === 'loaded' || state === 'complete') {
        link.onreadystatechange = null;

// #4   
var cssnum = document.styleSheets.length;
var ti = setInterval(function() {
    if (document.styleSheets.length > cssnum) {
        // needs more work when you load a bunch of CSS files quickly
        // e.g. loop from cssnum to the new length, looking
        // for the document.styleSheets[n].href === url
        // ...

        // FF changes the length prematurely :(
        CSSDone('listening to styleSheets.length change');
}, 10);

// MAGIC ends

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