Load CSS asynchronously
<link rel="preload" href="mystyle.css" as="style" onload="this.rel='stylesheet'">
<noscript><link rel="stylesheet" href="mystyle.css"></noscript>
<script src="loadcss.js"></script>
<script>
loadCSS('mystyle.css', 'all');
</script>
<script>
loadCSS('mystyle.css', 'all', function(link){
console.log(link.href);
});
</script>