This plugin integrates with and extends Autoptimize. It integrates with criticalcss.com, a premium service which uses a “headless” browser to extract real critical CSS. That way it fully automates the extraction of high-quality critical CSS and the creation of rules for that critical CSS. It can work 100% automated, but also allows semi-automated jobs (where you enter a URL for which you want a specific rule, the power-up will do the rest) and manual rules (where you create the rule and add critical CSS yourself).
Although current versions of Autoptimize can already tackle Google PageSpeed Insights’ “Eliminate render-blocking CSS in above-the-fold content” tip, the next release will allow you to do so in an even better way. As from version 1.9 you’ll be able to combine the best of both “inline CSS” and “defer CSS” worlds. “Defer” effectively becomes “Inline and defer“, allowing you to specify the “above the fold CSS” which is then inlined in the head of your HTML, while your normal autoptimized CSS is deferred until the page has finished loading.
Other improvements in the upcoming Autoptimize 1.9.0 include:
Inlined Base64-encoded background Images will now be cached as well and the threshold for inlining these images has been bumped up to 4096 bytes (from 2560).
Separate cache-directories for CSS and JS in /wp-content/cache/autoptimize, which should result in faster cache pruning (and in some cases possibly faster serving of individual aggregated files).
CSS is now added before the <title>-tag, JS before </body> (and after </title> when forced in head). This can be overridden in the API.
Some usability Improvements of the administration-page
Multiple hooks added to the API a.o. filters to not aggregate inline CSS or JS and filters to aggregate but not minify CSS or JS.
Multiple bugfixes & improvements
On the todo-list; testing, some translation updates (I’ll contact you translators in the coming week) and updating the readme.txt.
The first test-version of what will become 1.9.0 (still tagged 1.8.5 for now though) has been committed to wordpress.org’s plugin SVN and can be downloaded here. Anyone wanting to help out testing this new release, go and grab your copy and provide me with feedback.