> Are there really cases where the CSS is so complex or the page loads so many resources that this effort is worthwhile?
On the contrary, the more complex the css is or the more resources loaded, the less this would be worthwhile.
The thing i think they are trying to optimize is latency due to RTT. When you first request the html file, the browser needs to read it before knowing the next thing to request. This requires a round trip to the server, which has latency (pesky speed of light). The larger your (critical) css, the more expensive this optimisation is so the less likely it is a net benefit.
On the contrary, the more complex the css is or the more resources loaded, the less this would be worthwhile.
The thing i think they are trying to optimize is latency due to RTT. When you first request the html file, the browser needs to read it before knowing the next thing to request. This requires a round trip to the server, which has latency (pesky speed of light). The larger your (critical) css, the more expensive this optimisation is so the less likely it is a net benefit.