A function for loading CSS asynchronously [c]2015 @scottjehl, Filament Group, Inc. Licensed MIT
Place the loadCSS function inline in the head of your page (it can also be included in an external JavaScript file if preferable).
Then call it by passing it a stylesheet URL:
<head>
...
<script>
// include loadCSS here...
function loadCSS( href, before, media ){ ... }
// load a file
loadCSS( "path/to/mystylesheet.css" );
</script>
<noscript><link href="path/to/mystylesheet.css" rel="stylesheet"></noscript>
...
</head>By default, loadCSS will inject the new CSS stylesheet after the last stylesheet or script in the page. This should retain your CSS cascade as you'd expect.
before: By default, loadCSS attempts to inject the stylesheet link after all CSS and JS in the page. However, if you desire a more specific location in your document, such as before a particular stylesheet link, you can use thebeforeargument to specify a particular element to use as an insertion point. Your stylesheet will be inserted before the element you specify. For example, here's how that can be done by simply applying anidattribute to yourscript.
media: You can optionally pass a string to the media argument to set themedia=""of the stylesheet - the default value isall.
Onload listener support with link elements is spotty, so if you need to add an onload callback, include onloadCSS function on your page and use the onloadCSS function:
function onloadCSS( ss, callback ){ ... }
var stylesheet = loadCSS( "path/to/mystylesheet.css" );
onloadCSS( stylesheet, function() {
console.log( "Stylesheet has asynchronously loaded." );
});LoadCSS attempts to load a css file asynchronously, while maintaining the CSS cascade, in any JavaScript-capable browser. However, some older browsers will block rendering while the stylesheet is loading. This table outlines css loading support and async loading support.
| Browser | CSS Loads Successfully | CSS Loads without Blocking Render |
|---|---|---|
| Chrome Mac (latest and many recent versions) | Yes | Yes |
| Firefox Desktop (latest and many recent versions) | Yes | Yes |
| Opera Mac (latest and many recent versions) | Yes | Yes |
| Safari Mac (latest and many recent versions) | Yes | Yes |
| Safari iOS (latest and many recent versions) | Yes | Yes |
| Chrome Android 5.x | Yes | Yes |
| Chrome Android 4.x | Yes | Yes |
| Android Browser 2.3 | Yes | No |
| Kindle Fire HD | Yes | Yes |
| Windows Phone IE 8.1 | Yes | Yes |
| IE 11 | Yes | Yes |
| IE 10 | Yes | Yes |
| IE 9 | Yes | Yes |
| IE 8 | Yes | No |
| IE 7 | Yes | No |
| IE 6 | Yes | No |
We typically use loadCSS to load CSS files that are non-critical to the initial rendering of a site. See the EnhanceJS project Readme for examples of how we typically use it to improve page loading performance.
The reason this script is sometimes necessary is because there is no cross-browser means in HTML (currently at least) to load and apply a CSS file asynchronously. CSS references that use either link or import will cause browsers to block page rendering by default while their related stylesheet loads.
Both are very much appreciated - especially bug fixes. As for contributions, the goals of this project are to keep things very simple and utilitarian, so if we don't accept a feature addition, it's not necessarily because it's a bad idea. It just may not meet the goals of the project. Thanks!