How it's working
A web crawler follows the links of your site. On each page, we retrieve the CSS files and list the selectors. The selectors are tested on the html page. If the selector matches an element, it is marked as used. When all the files have been visited, a new css file with only the used selectors is built. That's the file you can download. It will also explore pages behind a login.
- Explore the pages of your site automatically, find the unused CSS selectors and create new clean CSS files you can download
- Authenticate and explore the pages requiring authentication. It will then check unused CSS rules.
- Customize the list of CSS selectors to keep
- Preview the change made to the CSS file
- Follow CSS import commands
- Explore responsive design rules, media queries
- Set a user agent for the crawler
Latish Sehgal has written a windows application to find and remove unused CSS classes. I haven't tested it but from the description, you have to provide the path of your html files and one CSS file. The program will then list the unused CSS selectors. From the screen-shot, it looks like there is no way to export this list or download a new clean CSS file. It also looks like the service is limited to one CSS file. If you have multiple files you want to clean, you have to clean them one by one.
Dust-Me Selectors is a Firefox extension (for v1.5 or later) that finds unused CSS selectors. It extracts all the selectors from all the style-sheets on the page you're viewing, then analyzes that page to see which of those selectors are not used. The data is then stored so that when testing subsequent pages, selectors can be crossed off the list as they're encountered. This tool is supposed to be able to spider a whole website but I unfortunately could make it work. Also, I don't believe you can configure and download the CSS file with the styles removed.
Topstyle is a windows application including a bunch of tools to edit CSS. I haven't tested it much but it looks like it has the ability to removed unused CSS selectors. This software costs 80 USD.
Liquidcity CSS cleaner is a php script that uses regular expressions to check the styles of one page. It will tell you the classes that aren't available in the HTML code. I haven't tested this solution.
Deadweight is a CSS coverage tool. Given a set of style-sheets and a set of URLs, it determines which selectors are actually used and lists which can be "safely" deleted. This tool is a ruby module and will only work with rails website. The unused selectors have to be manually removed from the CSS file.
CSSESS is a bookmarklet that helps you find unused CSS selectors on any site. This tool is pretty easy to use but it won't let you configure and download clean CSS files. It will only list unused CSS files.