With the large number of browser-based applications available, a new class within Software interface was needed. Prior to this change, there were four classes under Software interface: Graphical User Interface, Application Programming Interface, Command Line Interface and Web Service.
What we needed was a class which could represent a Web browser. This is more difficult than it sounds. A piece of software accessible via a browser has a GUI, but it’s not the same thing as a “standalone” application, which is what (in common usage) is called a GUI. Further, as Robert says, a distinction may need to be made at some point between Web 1.0 and Web 2.0 Web applications, and perhaps also Web forms. All Web browsers necessarily have a Web User Interface, which could be a superclass of some or all of these different browser-based interfaces.
Web User Interface can later be extended to also model these different sorts of web-based interfaces (via appropriate child classes, for example). It is worth noting that Web Service should probably not be a child of Web User Interface, as Web Service is more closely related to an API. For now we will not change the location of the Web Service class, and the only change is the addition of the Web User Interface as a sibling of the other four classes.