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How to load CSS asynchronously

Unlike JavaScript, CSS doesn’t have a native way to load it asynchronously. There’s no async or defer attributes for link elements the way there are for script elements.

In the past, I used to use a JavaScript helper function, loadCSS from Filament Group, for this.

But the smart folks at Filament have found a nearly native way to achieve asynchronous CSS loading, no helper function required.

Let’s check it out.

Why would you need this?

CSS is render blocking. While the browser is downloading it, it stops painting the document.

In some situations, it can make sense to asynchronously load non-critical CSS to minimize how long rendering is blocked. Some examples:

  1. If you’re minified and gzipped HTML + CSS is bigger than 14kb (about the size of a single roundtrip HTTP request), loading a subset of your CSS async can reduce render blocking.
  2. Since web fonts produce a flash of invisible text, we defer using them until they’re fully downloaded. This makes them a great candidate for async loading since they’re non-critical anyways.

Let’s look at how the technique works.

The newest technique from the Filament Group involves using a link element just like you normally would, but with two small changes.

First, set the media to print. Then, add an onload event to it that switches the media to all once the CSS file loads. For a bonus, you can also set the onload to null after.

<link rel="stylesheet" href="/path/to/my.css" media="print" onload="'all'; this.onload=null;">

Scott Jehl explains why the technique works in an article on the Filament site.

By declaring a media type that doesn’t match the current environment, we can achieve an interesting and useful effect: the browser will load the stylesheet without delaying page rendering, asynchronously!

So neat!

What about preload?

In response to yesterday’s article on self-hosting fonts, I had a bunch of people email asking about the preload attribute.

In my own experience and testing, this attribute actually slowed down rendering of pages for me. Filament’s article hints at why that may have been happening for me.

More importantly though, preload fetches files very early, at the highest priority, potentially deprioritizing other important downloads, and that may be higher priority than you actually need for non-critical CSS.

Your mileage may vary, of course. They do offer a fix in their article, but the newer approach above works great as-is.