wordpress-inline-css.php

<?php $cssupdate = '20150318'; ?>
<?php if ( isset($_COOKIE[ 'csscached']) && $_COOKIE[ 'csscached'] == $cssupdate ) : ?>
<link rel="stylesheet" href="<?php bloginfo('template_directory'); ?>/style.css?v=<?php echo $cssupdate; ?>">
<?php else : ?>
<style>
<?php include( bloginfo('template_directory') . '/css/critical.css'); ?>
</style>
<script>
/* Load critical CSS and cache stylesheet */
(function(win, doc) {
    'use strict';

    function loadCSS(href) {
        var ss = doc.createElement('link'),
            ref = doc.getElementsByTagName('script')[0],
            sheets = doc.styleSheets;
        ss.rel = 'stylesheet';
        ss.href = href;
        ss.media = 'only x';
        ref.parentNode.insertBefore(ss, ref);

        function toggleMedia() {
            var defined,
                i;
            for (i = 0; i < sheets.length; i = i + 1) {
                if (sheets[i].href && sheets[i].href.indexOf(ss.href) > -1) {
                    defined = true;
                }
            }
            if (defined) {
                ss.media = 'all';
            } else {
                win.setTimeout(toggleMedia);
            }
        }
        toggleMedia();
        return ss;
    }
    loadCSS(<?php bloginfo('template_directory'); ?> + '?v=<?php echo $cssupdate; ?>');
    doc.cookie = 'csscached=<?php echo $cssupdate; ?>;expires="Tue, 19 Jan 2038 03:14:07 GMT";path=/';
}(this, this.document));
</script>
<?php endif; ?>

Fork of https://gist.github.com/adactio/8a15bcdfef855bca16ed updated for Wordpress templates with query strings for cache busting opposed to the Apache rewrite