For this purpose Autoptimize hooks into template_redirect and will redirect to fallback Autoptimized CSS/ JS if a request for autoptimized files 404’s.
For cases where 404’s are not handled by WordPress but by Apache, AO adds an ErrorDocument directive in the .htaccess-file redirecting to wp-content/autoptimize_404_handler.php. Users on NGINX or MS IIS or … might have to configure their webserver to redirect to wp-content/autoptimize_404_handler.php themselves though, but those are smart cookies anyway, no?
I’m not a big fan of AMP but I do have it active here on this blog using the official AMP plugin for WordPress, using it in “Reader” (aka “classic”) mode. That’s as far as I want to take it, but suppose (as was the case for an Autoptimize/ Critical CSS user I was helping out) you want to redirect all mobile traffic to AMP, then you could use below code snippet to do just that.
The title says it all; I just pushed the first beta of Autoptimize 2.7 which has some small fixes/ improvements but which most importantly finally sees the “Autoptimize CriticalCSS.com power-up” fully integrated.
Next to the actual integration and switching to object-oriented for most (but not all) of AOCCSS files, there are some minor functional changes as well, most visible ones being buttons to clear all rules and to clear all jobs from the queue.
I hope to be able to release AO27 officially in April, but for that I need the beta thoroughly tested off course. Most important part to test is the critical CSS logic obviously, so if you have the power-up running, download/ install the beta and simply disable the power-up to have Autoptimize fill that void (if the power-up is active, AO refrains from loading it’s own critical CSS functionality).