This is a test page that references a stylesheet using a standard link[rel=stylesheet] with a print media type, which is swapped to all media when loaded..

That markup looks like this:

<link rel="stylesheet" href="slow.css" media="print" onload="this.media='all'">

Note: When run locally, the CSS file has a 5 second delay built into its server response time. If it is loaded in a non-blocking manner as desired, you should be able to read this text before the page is styled as white text on green background.