digitalapps

This plugins detects all of the styles queued via wp_enqueue_style. Contents of each css file are copied and stored in a single css file. This file is loaded on the frontend. The plugin wp_dequeue_style and wp_deregister_style all other css files. The result is your website only makes one single http request to load css styling. Bonus feature the contents of the css file are minified.
Simply flip the switch and the plugin will do the rest.
NOTE: I have only tested this plugin on my own sites running the AdSense Theme. If you encounter a problem with this plugin please let me know. I will be able to help you.

WP CSS Merge

This plugins detects all of the styles queued via wp_enqueue_style. Contents of each css file are copied and stored in a single css file.

launchinteractive

This plugin merges/concatenates Cascading Style Sheets & Javascript files into groups. It then minifies the generated files using Minify (for CSS) and Google Closure (for JS – fallback to Minify when not available). Minification is done via WP-Cron so that it doesn’t slow down the website. When JS or CSS changes files are re-processed. No need to empty cache!
Inspired by <a href="https://wordpress.org/plugins/minqueue/" rel="ugc">MinQueue</a> and <a href="https://wordpress.org/plugins/dependency-minification" rel="ugc">Dependency Minification</a> plugins.
Minification by <a href="https://github.com/matthiasmullie/minify" rel="nofollow ugc">Minify</a> and <a href="https://developers.google.com/closure/" rel="nofollow ugc">Google Closure</a>
In order to ensure fast loading times it’s recommended to set long expiry dates for CSS and JS as well as make sure gzip or deflate is on.
Version 1.10 added should_mmr filter which allows developers to disable MMR when needed. You could use this to disable MMR for certain WordPress roles or with some Page Builders. Three new plugins have been created that utilise this: 
<a href="https://wordpress.org/plugins/mmr-disable-for-divi" rel="ugc">Merge + Minify + Refresh Check DIVI</a>, <a href="https://wordpress.org/plugins/mmr-disable-for-visual-composer-editor" rel="ugc">Merge + Minify + Refresh Check Visual Composer</a> and <a href="https://wordpress.org/plugins/mmr-disable-for-beaver-builder-editor" rel="ugc">Merge + Minify + Refresh Check Beaver Builder</a>. Please install these when required.
Version 1.6 added support for HTTP2 Server Push to further speed up your requests (thanks to <a href="https://www.slightfuture.com" rel="nofollow ugc">Daniel Aleksandersen</a>).
Version 1.6.9 added the ability to specify the cache directory. Set MMR_CACHE_DIR & MMR_CACHE_URL constants in wp-config.php. MMR_CACHE_DIR must be full server path and MMR_CACHE_URL must be absolute URL for this to work correctly (thanks to <a href="https://www.slightfuture.com" rel="nofollow ugc">Daniel Aleksandersen</a>).
Version 1.6.11 added the ability to specify seperate cache urls for javascript and CSS. Use MMR_JS_CACHE_URL & MMR_CSS_CACHE_URL to replace MMR_CACHE_URL.
Version 1.7.0 added the ability to generate .css.gz & .js.gz files. Your webserver may need to be configured to use these files. Here is how to use these files in Apache (mod_rewrite, mod_deflate and mod_headers need to be enabled):
<pre><code><IfModule mod_rewrite.c>
 <IfModule mod_deflate.c>
 <IfModule mod_headers.c>
#Serve gzip compressed CSS files if they exist and the client accepts gzip.
RewriteCond %{HTTP:Accept-encoding} gzip
RewriteCond %{REQUEST_FILENAME}\.gz -s
RewriteRule ^(.*)\.css $1\.css\.gz [QSA]

#Serve gzip compressed JS files if they exist and the client accepts gzip.
RewriteCond %{HTTP:Accept-encoding} gzip
RewriteCond %{REQUEST_FILENAME}\.gz -s
RewriteRule ^(.*)\.js $1\.js\.gz [QSA]

#Serve correct content types, and prevent mod_deflate double gzip.
RewriteRule \.css\.gz$ - [T=text/css,E=no-gzip:1,E=is_gzip:1]
RewriteRule \.js\.gz$ - [T=text/javascript,E=no-gzip:1,E=is_gzip:1]
Header set Content-Encoding "gzip" env=is_gzip
 </IfModule>
 </IfModule>
</IfModule>
</code></pre>
Version 1.8.8 added a “merge_minify_refresh_done” hook that fires when JS or CSS has changed.
Version 1.11 added two new constants: 
MMR_USE_CLOSURE – allows Google Closure Javascript minification to be disabled. 
MMR_REMOVE_EXPIRED – disables removing expired files.
Version 1.13 added two new filters which can be used to exclude specific files: 
mmr_ignored_css_sources 
mmr_ingnored_js_sources 
Version 1.13 also automatically moves the WordPress inline styles to a global stylesheet which it then enqueues.
Version 1.14.3 added two new filters which can be used to modify the output before it gets saved: 
modify_css_output_before_save 
modify_js_output_before_save
Note Installing this plugin on a server with the eAccellerator module installed has the potential to break as Minify uses anonymous functions which return NULL. <a href="https://github.com/eaccelerator/eaccelerator/issues/12" rel="nofollow ugc">View eAccellerator Issue Thread</a>
Features
<ul>
<li>Merges JS and CSS files to reduce the number of HTTP requests</li>
<li>Handles scripts loaded in the header & footer</li>
<li>Compatible with localised scripts</li>
<li>Creates WP-Cron for minification as this can take some time to complete</li>
<li>Minifies JS with Google Closure (requires php exec) with fallback to Minify</li>
<li>Minifies CSS with Minify</li>
<li>Failed minification doesn’t break the site. Visitors will instead only see the merged results</li>
<li>Stores Assets in /wp-content/mmr/ folder</li>
<li>Uses last modified date in filename so any changes to JS or CSS automatically get re-processed and downloaded on browser refresh</li>
<li>View status of merge and minify on settings page in WordPress admin</li>
<li>Option to enable http2 server push (thanks to <a href="https://www.slightfuture.com" rel="nofollow ugc">Daniel Aleksandersen</a>)</li>
<li>Option to enable output buffering for compatibility and so footer scripts can be HTTP2 pushed</li>
<li>Ability to turn off minification</li>
<li>Ability to turn off concatenation</li>
<li>Ability to manually ignore scripts or css</li>
<li>Ignores conditional scripts and styles</li>
<li>Ability to specify cache directories</li>
<li>Ability to generate .css.gz & .js.gz files (Thanks to Marcus Svensson)</li>
<li>Works with WordPress Multisite</li>
</ul>

Merge + Minify + Refresh

I just tested this on localhost not on a production site but its seems like it works like a charm and does its jobs really well.

So far i can't see any script or style conflicts, so i guess on a good codebase (probably when all scripts and styles are integrated via WordPress enqueue API) this plugin just works.

Im gonna test the impact on a live website but i think this is gonna be quite relevant.

Seems really powerfull

Great plugin. Works as expected.
Thank you.

Great plugin.

Merges/Concatenates CSS & Javascript and then minifies using Minify (for CSS) and Google Closure (for JS with Minify as a fallback).

WP CSS Merge vs Merge + Minify + Refresh