an experimental option to provide “fallback autoptimized CSS and JS”; in some cases a cached version of the HTML links to autoptimized CSS/ JS that was already removed. if you enable that option, Autoptimize will try to intercept those requests and redirect them to fallback versions to limit the breakage. the option is default off for now, but might become default on at a later stage.
Yesterday evening, not being able to sleep, I was going through my YouTube subscriptions and stumbled on this James Blake Corona lockdown livingroom-concert on friggin’ Instagram, of well over an hour. Goosebumps!
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.