[zaphodias]

isn't the whole point avoiding FOUC, while also avoiding to block the rendering for CSS network requests?

[lxe]

Unless you're sure that your the "non-critical" css doesn't cause layout shifts (aka, it doesn't overload any "critical" styles), you're gonna see layout shifts even on fast connections if you load some styles at the top of the document and then do a link rel at the bottom.

[rtsil]

The critical css should cover everything above the fold to avoid that visible reflow.