The upside down of the traditional thought on user interface

User interface (UI, or human-computer interface) is a central concept in modern software development. Essentially, UI provides the way how humans may interact with computers. Given the World Wide Web as the background, UI represents how the real world interacts to the virtual world and vice versa.

There is an interesting question about UI on the Web. Should the UI be an external component to its software or be an internal component?

Sound silly? In tradition, an UI is unquestionably an external component with respect to its software. We may envision UI to be the cover wraps its internal executable programs. However, does it mean that UI must be in this form?

Let us turn to a natural phenomenon. We humans live in this world (as well as many other creatures). In fact, every one of us may see ourselves as an independent world that is interacting with the shared external world, which composes of all the natural resources such as air and water. Therefore, there is an interface between the two "worlds". As a matter of fact, however, much (if not all of) the UI is located internally with respect to our own world (i.e., our body) such as lung (exchanging air) or stomach (exchanging water). The external components of our world (such as skin) are essentially for protection and identification besides few supporting on interacting to the external world. Such an analogy leads to an interesting question, could UI on the Web also primarily be an internal component in contrast to an external component to the respective Web applications? Maybe this one is just too silly a question to ask since the non-external ones on the Web cannot be an UI by definition. Then, what is Web UI really? May somebody give a better answer in the philosophical sense but not the technological sense?