From Xojo Documentation
| This property is only available on the Windows platform.
For cross-platform compatibility, use ＃If...＃Endif with the Target... specifiers to make sure you will not attempt to use this property on an incompatible platform.
Used to specify the rendering engine used on Windows. Choices are Native (0) or WebKit (1).
Choosing WebKit includes the WebKit engine in your built application, increasing the size of your application by about 20MB.
If you need plugins when using WebKit, add them to the WebKitPlugins folder in the Libs folder for your application.