How to load up CSS files using Javascript?

Question

Here's the "old school" way of doing it, which hopefully works across all browsers. In theory, you would use setAttribute unfortunately IE6 doesn't support it consistently.

var cssId = 'myCss';  // you could encode the css path itself to generate id..
if (!document.getElementById(cssId))
{
    var head  = document.getElementsByTagName('head')[0];
    var link  = document.createElement('link');
    link.id   = cssId;
    link.rel  = 'stylesheet';
    link.type = 'text/css';
    link.href = 'http://website.com/css/stylesheet.css';
    link.media = 'all';
    head.appendChild(link);
}

This example checks if the CSS was already added so it adds it only once.

Put that code into a javascript file, have the end-user simply include the javascript, and make sure the CSS path is absolute so it is loaded from your servers.

VanillaJS

Here is an example that uses plain JavaScript to inject a CSS link into the head element based on the filename portion of the URL:

<script type="text/javascript">
var file = location.pathname.split( "/" ).pop();

var link = document.createElement( "link" );
link.href = file.substr( 0, file.lastIndexOf( "." ) ) + ".css";
link.type = "text/css";
link.rel = "stylesheet";
link.media = "screen,print";

document.getElementsByTagName( "head" )[0].appendChild( link );
</script>

Insert the code just before the closing head tag and the CSS will be loaded before the page is rendered. Using an external JavaScript (.js) file will cause a Flash of unstyled content (FOUC) to appear.

If you use jquery:

$('head').append('<link rel="stylesheet" type="text/css" href="style.css">');

I guess something like this script would do:

<script type="text/javascript" src="/js/styles.js"></script>

This JS file contains the following statement:

if (!document.getElementById) document.write('<link rel="stylesheet" type="text/css" href="/css/versions4.css">');

The address of the javascript and css would need to be absolute if they are to refer to your site.

Many CSS import techniques are discussed in this "Say no to CSS hacks with branching techniques" article.

But the "Using JavaScript to dynamically add Portlet CSS stylesheets" article mentions also the CreateStyleSheet possibility (proprietary method for IE):

<script type="text/javascript">
//<![CDATA[
if(document.createStyleSheet) {
  document.createStyleSheet('http://server/stylesheet.css');
}
else {
  var styles = "@import url(' http://server/stylesheet.css ');";
  var newSS=document.createElement('link');
  newSS.rel='stylesheet';
  newSS.href='data:text/css,'+escape(styles);
  document.getElementsByTagName("head")[0].appendChild(newSS);
}
//]]>

Element.insertAdjacentHTML has very good browser support, and can add a stylesheet in one line.

document.getElementsByTagName("head")[0].insertAdjacentHTML(
    "beforeend",
    "<link rel=\"stylesheet\" href=\"path/to/style.css\" />");

If you want to know (or wait) until the style itself has loaded this works:

// this will work in IE 10, 11 and Safari/Chrome/Firefox/Edge
// add ES6 poly-fill for the Promise, if needed (or rewrite to use a callback)

let fetchStyle = function(url) {
  return new Promise((resolve, reject) => {
    let link = document.createElement('link');
    link.type = 'text/css';
    link.rel = 'stylesheet';
    link.onload = function() { resolve(); console.log('style has loaded'); };
    link.href = url;

    let headScript = document.querySelector('script');
    headScript.parentNode.insertBefore(link, headScript);
  });
};

In a modern browser you can use promise like this. Create a loader function with a promise in it:

function LoadCSS( cssURL ) {

    // 'cssURL' is the stylesheet's URL, i.e. /css/styles.css

    return new Promise( function( resolve, reject ) {

        var link = document.createElement( 'link' );

        link.rel  = 'stylesheet';

        link.href = cssURL;

        document.head.appendChild( link );

        link.onload = function() { 

            resolve(); 

            console.log( 'CSS has loaded!' ); 
        };
    } );
}

Then obviously you want something done after the CSS has loaded. You can call the function that needs to run after CSS has loaded like this:

LoadCSS( 'css/styles.css' ).then( function() {

    console.log( 'Another function is triggered after CSS had been loaded.' );

    return DoAfterCSSHasLoaded();
} );

Useful links if you want to understand in-depth how it works:

Official docs on promises

Useful guide to promises

A great intro video on promises