Even if the rest of the CSS files take seconds to load, these specific CSS styles displayed from the head section ensure quick rendering of your page and better score in testing systems like Google Page Speed Insights..
But you do not need to maintain two separate stylesheets, just select specific fragments of your theme style file with markup to create above-the-fold styles joining the CSS fragments in the head section..
The easiest way, the CSS is rendered in the head but not in the stylesheet..
The unobtrusive way, it allows you to include comments inside fragments, but the fragments are rendered both in the head section and the CSS file..
This plugin read the style css file when it's modified, and stores the results using WP options API...
Activate the plugin through 'Plugins WordPress.
If you want stylesheet use Comment only the tags technique...
The aim is simplicity, editing single file avoids having to maintain two separate stylesheets...
There are some tools to help you in the process of select the rights fragments of code, and maybe you will have to modify your stylesheet structure to isolate these CSS fragments..
Browse the code, check out SVN repository, or subscribe to the development log by RSS...
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