
If you're working in a company with closed-source (proprietary) software, I'll suggest not using an online service to compress your CSS code. In that case, use an in-house compressor. Let's get started, now!
1. Code Beautify - This one is my favorite and that's why it is first on this list. It processes the CSS code instantly and shows it in a colored and formatted way on the right side. You can easily compress it too with a single click of a button.
All the 3 common ways of acquiring the CSS code are supported, viz., direct code pasting, file upload, and URL of a webpage. It also has a handy download button for the beautified CSS code.
2. Clean CSS - The name says it all! It's a minimal and blazing fast CSS beautifier tool. Even large files with a lot of CSS code are processed in a quick time.
Code syntax highlighting is fully supported so that you can visualize the formatted CSS code in the best possible way. I've used this tool for JavaScript formatting too. It's definitely worth giving a try.
3. Beautify Tools - This one offers several unique features. Before beautifying your CSS code, you can test its result with dummy data. It can be done with a click of a button.
One can select the font size as well as the syntax highlighting theme for the result pane. You can either copy the CSS code to the clipboard or can download it in the form of a file.
4. Free Formatter - Like all other similar options, this one too supports both a file upload as well as a direct copy-paste. It also has the option to provide the character encoding format which defaults to UTF-8.
The indentation level for the tab size can also be customized s per your preferences. This ensures the code is formatted in your preferred way. You can also opt to get the result in a new window.
5. CSS Portal - This excellent service offers both the beautification as well as minification of CSS code in the same window. There's no need to open two different pages for both.
As usual, both the clipboard copy and file download are supported. One can also clear the input text field with a click of a button. Do once give it a try.
6. CSS Beautifier - It has a beautiful and minimal tabbed-based interface that offers processing options both for CSS and JavaScript code. You can easily switch between tabs, at will.
The result is syntax highlighted so that you can visualize the formatting, easily. Though this service is last on the list, it's equally capable of beautifying your CSS code with a click of a button.