This library is for a quick optimization of the CSS rules of an AMP document, eliminating 95% of the rules that are not being used in the current document.
I tried some approaches to get the fastest algorithm, and now it's really fast, so you can use it in every page of your website.
More information about AMP: https://www.ampproject.org/
The operation is simple: it is enough to process the final document in AMP format, including the <style amp-custom> tag and the body tag.
Version 2.0 have been completely rebuilt, and now supports @media rules, removing them completely when they are no longer needed.
It also respects the @keyframes leaving them unchanged.
If you want to preserve some CSS rules, you can also tell the library to keep them.
First include the file amp_remove_css.class.php
$tmp = new AmpRemoveUnusedCss();
$tmp->preserve_selectors[] = ".please-dont-delete-me"; // preserve this css rule anywhere it appears
$tmp->process($htmlcode); //must be full htmlcode, with <style amp-custom> tag and the <body> content
echo $tmp->result();
$tmp = new AmpRemoveUnusedCss(1); //set 1 or TRUE to get full report, or void or 0 or FALSE to get simple report
$tmp->process($htmlcode);
echo $tmp->report();
You also can just only minify CSS by calling (it removes useless white spaces, but it does not remove unused CSS rules):
$tmp = new AmpRemoveUnusedCss();
$css_minified = $tmp->minify($css);