hmmm, AO should preload all CSS that is loaded in the HTML that is loaded with <link rel='stylesheet' but if Thrive or Customer reviews load their CSS differently (?) then you would indeed need to look for a different solution?
Ok, thanks for the quick reply! I’ll check with both of them to see how CSS is loaded. Here are the files in question in case this means anything to you:
/wp-content/plugins/thrive-leads/editor-layouts/css/frontend.css?ver=2.2.6.2
/wp-admin/load-styles.php?c=1&dir=ltr&load%5B%5D=dashicons,wp-jquery-ui-dialog&ver=5.2.4
/wp-content/plugins/wp-customer-reviews/css/wp-customer-reviews-generated.css?ver=3.4.1
/wp-content/mu-plugins/wpengine-common/css/wpe-common.css?ver=3.2.2
The CSS URL’s are not very revealing, but if you can share your site’s URL I’d be happy to have a look 🙂
No, don’t see those referenced in the HTML, so AO does not “see” them. They are probably loaded by some JS, which also explains why those files are loaded so late as can be seen in this screenshot of a webpagetest.org test;

Given these files load that late and are loaded by (deferred) JS instead of HTML, I don’t think those can be considered render-blocking really, which is confirmed by the 0.5s FCP in the same test (URL here)
Good to know. Thanks for checking that out. 🙂