Hi Frank, hope you're well,
I was wondering in what sorts of environments/situations/instances you've been able to get the filter applicable to inlining different 'above-the-fold', critical CSS for different pages (or posts) to work without 'breaking' a site.
I ask as since the AO Inline and Defer feature was released, we've run into precisely zero instances in which trying to use the filter applicable to rendering different critical CSS on different pages doesn't break everything. I'm all but certain we've tried every possible permutation with it at this point, too: from creating separate plugins, to using Code Snippets, to applying the filter in functions.php; minifiying the CSS within the filter, not minifying the CSS within the filter... It just doesn't seem to work, mate.
Best,
AJ
P.s: As an aside: I'd pay you just about anything you want to develop the capacity to apply different critical CSS to different pages right from within the AO UI, as it would still likely be cheaper than having an entirely novel plugin developed that accomplishes this, which is what we're currently exploring.