I'm looking for a free/gratis (online or offline) CSS 3 Editor.
I found this one http://www.cssportal.com/online-css-editor but it is way too basic.
Plataforms: online / windows.
Software Recommendations Stack Exchange is a question and answer site for people seeking specific software recommendations. It only takes a minute to sign up.Sign up to join this community
Backets is an open source HTML, JS and CSS editor sponsored by Adobe. It's clean and comfortable to use if you already know CSS. It includes a "live preview" option and can extract design information from PSD files. There are also extensions for you to add extra features.
You have given so little information that it is difficult to advise you.
However, you can hardly go wrong with Amaya, since it's a project from the The World Wide Web Consortium (W3C) and so shines in the area of standards compliance and accessibility.
Work on Amaya started at W3C in 1996 to showcase Web technologies in a fully-featured Web client. The main motivation for developing Amaya was to provide a framework that can integrate as many W3C technologies as possible. It is used to demonstrate these technologies in action while taking advantage of their combination in a single, consistent environment.
Amaya started as an HTML + CSS style sheets editor.
I would suggest you: