While editing a file, Google Web Designer uses -webkit prefixes. However, when you publish your content, the publish dialog allows you to specify additional vendor prefixes (or no prefixes) in the output so the content works in different browsers.
I hope this helps.
(Google Web Developer)
Google Web Designer (GWD) uses the WebKit HTML rendering engine inside the application's workspace and reports the styles that are actually applied to the content during editing.
Google Web Designer team