They do, you just need to select a profile in the "Advanced Options" section. The default level is CSS 2.1, which is the most comprehensive and final standard at the moment. CSS level 3 is still in the early stages of development - meaning that any part of the specification can change a lot - and even the validator can sometimes make mistakes, so it hasn't become a default yet.
Keep in mind that styles with a vendor prefix will never be checked because they are non-standard , but since Juhana says you can at least have trigger warnings for your CSS to conditionally check.