How to load CSS Asynchronously

html css asynchronous
Paulina994 picture Paulina994 · Sep 24, 2015 · Viewed 98.9k times · Source

I'm trying to eliminate 2 CSS files that are render blocking on my site - they appear on Google Page Speed Insights. I have followed different methods, none of which were a success. But, recently, I found a post about Thinking Async and when I applied this code: <script async src="https://third-party.com/resource.js"></script>it did eliminate the problem.

However, after publishing, the page lost the styling. I'm not too sure as to what is going on because the code works but it's the styling after upload that doesn't work. Would appreciate your help with this. Thanks

Answer

Vladimir Salguero picture Vladimir Salguero · Oct 29, 2016

The trick to triggering an asynchronous stylesheet download is to use a <link> element and set an invalid value for the media attribute (I'm using media="none", but any value will do). When a media query evaluates to false, the browser will still download the stylesheet, but it won't wait for the content to be available before rendering the page.

<link rel="stylesheet" href="css.css" media="none">

Once the stylesheet has finished downloading the media attribute must be set to a valid value so the style rules will be applied to the document. The onload event is used to switch the media property to all:

<link rel="stylesheet" href="css.css" media="none" onload="if(media!='all')media='all'">

This method of loading CSS will deliver useable content to visitors much quicker than the standard approach. Critical CSS can still be served with the usual blocking approach (or you can inline it for ultimate performance) and non-critical styles can be progressively downloaded and applied later in the parsing / rendering process.

This technique uses JavaScript, but you can cater for non-JavaScript browsers by wrapping the equivalent blocking <link> elements in a <noscript> element:

<link rel="stylesheet" href="css.css" media="none" onload="if(media!='all')media='all'"><noscript><link rel="stylesheet" href="css.css"></noscript>

You can see the operation in www.itcha.edu.sv

enter image description here

Source in http://keithclark.co.uk/

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