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.
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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.