The critical_min.css already contains styles relating to only content ‘above the fold’, hence the inclusion of ‘min’ in the name of the css. As for the JS loader you refer to, I believe it may be preferable to add a .js that hides the display until all remaining css are loaded to resolve the FOUC issue. Once again, as the FOUC occurs for only a split second when a page is first loaded from the server, one thing I’ve recently noticed is that my page speed on a mobile device has been substantially increased going from 5c to less then 2s in most cases. As my page speed on a desktop was already well below 2-3 seconds or in the 1.6 second range, my thinking is that the page speed on a mobile device is becoming more important in that more and more people are using their mobile phones not to mention that Google has been saying for a long time that they eventually anticipate giving more weight to how a site performs on a mobile device as opposed to a desktop for ranking in the serps. If true, the trade-off in having a split second FOUC in viewing my website on a desktop seems moot at this point in time since the behavior doesn’t occur on my mobile phone device. What say u?