I'm hoping you mean tool -- as there is no price at the gate. The validator sensei.design linked to is the official one... Though word of warning. While there is little if any valid excuse for invalid/non-validating markup, CSS validation is a pipe-dream in the age of vendor specific prefixes to implement CSS3 and the sane use of IE specific fixes like Zoom. These do not validate, but are accepted practice in the web development community. As such getting your CSS to validate is not as important as knowing WHY something is throwing an error. Having the knowledge to dismiss a pointless CSS error is important, as is recognizing when a REAL error is causing a problem. That said the page in question has 17 markup errors to be fixed before even THINKING about CSS problems. Of course that it's a train wreck of inaccessible javascript for nothing garbage, with nonsensical non-semantic markup and gibberish use of numbered headings means it's got a LONG ways to go.
Not to mention all the duplication - 2 message blocks with the same messages, three different methods of handling AJAX ... It looks as if it was cobbled together out of a few different sites, by someone with very little programming knowledge.