neoslab

Powered Minifier allow you to reduce your page load and increase your performance by minifying your HTML source along with all the CSS and JS code present in your markup.
<h3>License</h3>
Good news, this plugin is free for everyone! Since it’s released under the GPL, you can use it free of charge on your personal or commercial site.

Powered Minifier

Honestly it's the best minifier plugin for Wordpress 

Simply Excellent

Reduce your page load by minifying your HTML source along with all the CSS and JS code present in your markup.

peterpfeiffer

Minify and Optimize your CSS by clicking one button. All you have to do is go to Settings and click CSS Minify. Activating the plugin there will minify and optimize your CSS. No settings required, just click one button.
So anything you can do to speed up the load time of your site should be worth investigating, right? One such thing is “minifying” the CSS files that are used in part to build your site. As is typically the case, there is a plugin available that can do the job for you – Suprise: It’s this one!
Code from this plugin is forked from Autoptimize a really great plugin. You can find it by searching for it here on wordpress.org in the plugin search.
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Upload the zip file and unzip it in the /wp-content/plugins/ directory.
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Activate the plugin through the Plugins menu in WordPress
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Go to Settings and then click CSS Minify and enable the options you want.
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CSS Minify

Way to go, peterpfeiffer! Your plugin contains the same files and functions as in Autoptimize and your plugin's PRO version provides what people could get for free in Autoptimize. Who knew "Copy, cut and paste" could be a business model?!

Autoptimize, but with most features moved to PRO

Minify and Optimize your CSS by clicking one button.

Powered Minifier vs CSS Minify