flymake-csslint.el is an Emacs integration of CSSLint. The implementation is very much based on flymake-jshint.el by Wilfred Hughes.
The easiest way to install flymake-csslint is probably to install it via the ELPA archive at Marmalade.
ELPA (package.el) is part of Emacs 24. For Emacs 23 see Marmalade for installation instructions.
If you don't install via ELPA make sure that flymake-csslint.el is in your load-path and require the library
(add-to-list 'load-path "~/.emacs.d/path/to/flymake-csslint")
(require 'flymake-csslint)
To use CSSLint with emacs, you will need to have a command line version of CSSLint installed, see installation instructions at https://github.com/stubbornella/csslint/wiki/Command-line-interface.
If the csslint executable is not available on your PATH your need to
configure the flymake-csslint-program variable and let it point to the
executable.
M-x customize-variable <RET> flymake-csslint-program <RET>
If CSSLint isn't working for any reason, execute
M-x set-variable <RET> flymake-log-level <RET> 3 <RET>
and you will see what is going wrong listed in the Messages buffer.
flymake-csslint.el is developed at GitHub. Feature requests, ideas, bug reports, and pull request are more that welcome!