I posted earlier in this thread that this looks like the first time you can use Segoe UI legally in a web app: that's wrong. Fabric CSS doesn't actually include the webfonts.
https://github.com/OfficeDev/Office-UI-Fabric/blob/master/gh...
https://github.com/OfficeDev/Office-UI-Fabric/blob/master/gh...
Perhaps I'm missing something, but that makes no practical sense to me. Typography.com's model of per site licensing is much more understandable.