You could have downloaded Tart 0.9.1 for Mac from our software library for free. The application is included in System Tools. This Mac app was originally created by Illumit L.L.C.
From the developer:
Tart is a free, standalone, CSS and markup tester with a built in editor.
Tart is a unique Web validator. It is a standalone application so validating local files with it is easy. It can validate CSS, XHTML (including embedded CSS), and Atom/RSS feeds. Finally, it has a built in editor so you can repair problems without changing applications.
You may want to check out more software for Mac, such as Notational Velocity or ScrapPad, which might be similar to Tart.