Scott Jehl of Filament Group demonstrates a one-liner technique for loading external CSS files without them delaying page rendering.
While this isn’t really necessary in situations where your (minified and compressed) CSS is small, say 14k or below, it could be useful when working with large CSS files and want to deliver critical CSS separately and the rest asynchronously.
Today, armed with a little knowledge of how the browser handles various link element attributes, we can achieve the effect of loading CSS asynchronously with a short line of HTML. Here it is, the simplest way to load a stylesheet asynchronously:
<link rel="stylesheet" href="my.css" media="print" onload="this.media='all'">Note that if JavaScript is disabled or otherwise not available your stylesheet will only load for print and not for screen, so you’ll want to follow up with a “normal” (non-print-specific) stylesheet within <noscript> tags.