The latest versions of Windows 10 and Windows Phone indeed support CardDAV and CalDAV - the open sync standards that fruux relies on.

But so far Microsoft does not officially support this feature sadly (hopefully they will in a future version). But using a little workaround it's possible to setup Windows 10 and Windows Phone for use with fruux.

This workaround is described in the following Reddit post for Windows Phone and works similar for Windows 10: