There are many tools (freewares) available, depending on what in CSS you are taked with testing. For syntax checking I recommend CSS Lint as it provides you with a command line syntax check. Image Diff tools can come in handy for an overall comparison of the page from one test cycle to another, this however is recommended only for shorter cycles. Another tool for CSS testing would be PhantomCSS, as it provides is a version of WebKit that can run from the command line and be controlled via JavaScript – not just the usual in-page JavaScript like ‘pop open an alert box’ but browserlevel control such as, ‘Open a new tab, go to www.netmagazine.com, click on the tutorial’s link’. PhantomCSS harnesses this to perform image diff testing. In between PhantomCSS and PhantomJS, there’s actually another layer designed to either simplify things or complicate them, depending on your point of view. That layer is CasperJS, an abstraction layer and testing tool that sits on top of PhantomJS. Fortunately, that comes bundled with PhantomCSS. For UI testing I have always been fan of automated sanity scripts, which really have a grand coverage but the least trun-around time. You can use selenium to quickly spin some great UI sanity tests with multi-browser support.
