Hi,
We added this feature a few versions back, but it has been since then removed and it’s now under development for the future.
Basically, I removed the CSS async option, because async doesn’t preserve the order of the css files, and most people end up with more than 1 css file generated.
Async would make sense, only if you have a single, merged CSS optimized file per page (plus the critical path css), else css that was originally defined later can end up loading earlier and be overwritten by another async css file (it’s random, you cannot predict which loads first).
I’m still looking into this for the future, but if you’re doing critical path optimization, you could “collect all css and then do uncss with gulp / grunt instead of looking for the critical path”. That however, not so easy for a non developer.