I added the attribute rel="preload" to all css links like this :
<link rel='preload' onload='this.rel="stylesheet"' as='style'
id='reworldmedia-style-css' href='style.css' type='text/css' media='all'
/>
It works fine in Chrome but not in Safari or Firefox
I found a possibly best solution is to load two files as below - browsers that support preload will use it as intended and those that don't (like Firefox) will only use the regular (2nd link). This solution doesn't require using the onload="this.rel='stylesheet'" since the style is used right after the preload:
<head>
<link rel="preload" href="style.css" as="style">
<link rel="stylesheet" href="style.css">
</head>
What I also discovered is a hacky alternative to the above could be including "rel" twice, like:
<link href="style.css" rel="stylesheet" rel="preload" as="style">
For Firefox, it's only supported in Firefox 56 Nightly. It will ship on September 26, 2017. You can download it from here.
Update: This feature is landed on FF 56 but removed in 57. Here is the reason:
This feature was available in Firefox 56, but only for cacheable resources. It has been disabled in Firefox 57 because of various web compatibility issues (e.g. bug 1405761). An improved version that works for non-cacheable resources is expected to land in Firefox 59
See can I use.
It is not supported in Firefox and will be added in the next release of Safari.
So what you are seeing is expected behaviour.
I can't think of something more explanatory than the documentation itself. On the caniuse.com site there is this http://caniuse.com/#feat=link-rel-preload and if you follow that and go to the w3c specifications you find this. https://w3c.github.io/preload/ where is clearly stated that "This is a work in progress and may change without any notices." Maybe soon when this "Draft" will be refined, support will be added.
Reliably preload across browsers with loadCSS
One issue with preloading is that it is not supported by browsers like Firefox and Internet Explorer (as of Nov 2018). As a result, these browsers will not download the CSS file and this can result into serious display issues for your web-pages. This makes it critical to include a JavaScript polyfill for preload - loadCSS.
<style>
[Your critical CSS goes here]
</style>
<!– Reliably defer loading of non-critical CSS with loadCSS–>
<link rel=“preload” href=”[yourcssfile]” as=“style” onload=“this.onload=null; this.rel=‘stylesheet’”>
<noscript><link rel=“stylesheet” href=”[yourcssfile]” ></noscript>
<!– Inline the loadCSS preload polyfill code or load it via external JS file–>
<script src=“./cssrelpreload.js”></script>This works <link rel="preload stylesheet" href="./style.css" as="style"> to instruct the browser to download key resources as soon as possible.
