How we make RWD sites load fast as heck
Scott shares the code that Filament Group are using to determine which style declarations are critical (and can be inlined) and which are non-critical (and can be loaded asynchronously). It makes quite a difference in perceived performance.
By the way, I really, really like the terminology of “critical” and “non-critical” CSS, rather than “above the fold” and “below the fold” CSS.