The deferred font loading logic for Smashing Magazine. http://www.smashingmagazine.com/

sm-annotated.html

<script type="text/javascript">
    (function () {
      "use strict";
      // once cached, the css file is stored on the client forever unless
      // the URL below is changed. Any change will invalidate the cache
      var css_href = './index_files/web-fonts.css';
      // a simple event handler wrapper
      function on(el, ev, callback) {
        if (el.addEventListener) {
          el.addEventListener(ev, callback, false);
        } else if (el.attachEvent) {
          el.attachEvent("on" + ev, callback);
        }
      }
      
      // if we have the fonts in localStorage or if we've cached them using the native batrowser cache
      if ((window.localStorage && localStorage.font_css_cache) || document.cookie.indexOf('font_css_cache') > -1){
        // just use the cached version
        injectFontsStylesheet();
      } else {
       // otherwise, don't block the loading of the page; wait until it's done.
        on(window, "load", injectFontsStylesheet);
      }
      
      // quick way to determine whether a css file has been cached locally
      function fileIsCached(href) {
        return window.localStorage && localStorage.font_css_cache && (localStorage.font_css_cache_file === href);
      }

      // time to get the actual css file
      function injectFontsStylesheet() {
       // if this is an older browser
        if (!window.localStorage || !window.XMLHttpRequest) {
          var stylesheet = document.createElement('link');
          stylesheet.href = css_href;
          stylesheet.rel = 'stylesheet';
          stylesheet.type = 'text/css';
          document.getElementsByTagName('head')[0].appendChild(stylesheet);
          // just use the native browser cache
          // this requires a good expires header on the server
          document.cookie = "font_css_cache";
        
        // if this isn't an old browser
        } else {
           // use the cached version if we already have it
          if (fileIsCached(css_href)) {
            injectRawStyle(localStorage.font_css_cache);
          // otherwise, load it with ajax
          } else {
            var xhr = new XMLHttpRequest();
            xhr.open("GET", css_href, true);
            // cater for IE8 which does not support addEventListener or attachEvent on XMLHttpRequest
            xhr.onreadystatechange = function () {
              if (xhr.readyState === 4) {
                // once we have the content, quickly inject the css rules
                injectRawStyle(xhr.responseText);
                // and cache the text content for further use
                // notice that this overwrites anything that might have already been previously cached
                localStorage.font_css_cache = xhr.responseText;
                localStorage.font_css_cache_file = css_href;
              }
            };
            xhr.send();
          }
        }
      }

      // this is the simple utitily that injects the cached or loaded css text
      function injectRawStyle(text) {
        var style = document.createElement('style');
        // cater for IE8 which doesn't support style.innerHTML
        style.setAttribute("type", "text/css");
        if (style.styleSheet) {
            style.styleSheet.cssText = text;
        } else {
            style.innerHTML = text;
        }
        document.getElementsByTagName('head')[0].appendChild(style);
      }

    }());
</script>

I had an issue with IE9+ that it didn't show my fonts (the css was loaded successfully) on the first request (with an empty localstorage). Looks like IE doesn't render the lately injected style tag.
Is anybody experiencing the same?
I "fixed" it by adding an empty style tag to my html and filling this tag in the injectRawStyle function instead of adding a new tag.

Update: I found a better fix at http://www.phpied.com/dynamic-script-and-style-elements-in-ie/
It's important for IE to append the style tag before setting it's content! And it works!

@hdragomir if the problem is unreliable browser caching of fonts, why not just base64-encode your fonts in CSS and load that as normal (or asynchronously), skipping localStorage altogether? Or should we consider all browser caching to be unreliable, and use localStorage for all long-term static assets?

I've added a check to see if the XMLHTTPRequest response status code is 200, as it was caching a 404 page when the CSS file didn't get deployed.

Forked, updated and tested:
https://gist.github.com/iamdarrenhall/53b5eea54ac4bc25d0a6

Or just change line 54 to:

if (xhr.readyState === 4 && xhr.status === 200) {

I've done another update to prevent IE8 crashing. Moved the document.getElementsByTagName('head')[0].appendChild(style); above the style.styleSheet.cssText = text;

https://gist.github.com/iamdarrenhall/53b5eea54ac4bc25d0a6

@iamdarrenhall + @adamhavel: which is pretty weird, when you think about it. after all, it was IE who introduced us to innerHTML.

cu, w0lf.

Is there a way to do the same with just font files without combining them into a base encoded CSS file? Some web-font providers unfortunately don't allow to do that even if you're self hosting.

Take a look at my version, which is much more versatile: https://github.com/kurtextrem/localFont/blob/gh-pages/launcher.html
(I'd recommend to use a conditional for IE8 instead of loading it through the script though)