2021 edit:
Original link moved – async CSS without JavaScript
https://codepen.io/tigt/post/async-css-without-javascript
“It seems this trick causes Chrome & Firefox to start the body earlier,
and they simply don’t block for body stylesheets.”
<head>
<!--[if IE]>
<link rel="stylesheet" href="style.css">
<![endif]-->
</head>
<body>
<!--[if !IE]> -->
<link rel="stylesheet" href="style.css" lazyload>
<!-- <![endif]-->
</body>
The article also contains benchmarks:
