In this paper we deal with semantics issues related to the stability, navigability and extensibility of web applications. To this end, we define a web application as a deterministic labelled transition system in which states are the html pages and transitions are urls. This definition allows us, on the one hand, to characterize the temporal evolution of a web application and, on the other, to classify web applications into several types according to how the information is spread over the web application. This last classification captures interesting semantic properties related to the navigability and web application extensibility. We use partial orders to define and characterize web applications extensibility as web program refinements in a way that preserve navigability and improve the information reached through browsing. We apply these ideas to construct a simple web application, namely, a small virtual museum based on approximations of three original paintings. Despite of being very simple, this example evidences the need for the semantic properties while designing the web application in order to assure navigability and obtaining better information while browsing. Based on the extensibility characterization, we were able to extend the virtual museum with different paintings approximations while preserving navigability properties as well as browsing of paintings' approximations of increased quality resolution. Finally, based on our model for web applications we formally define the web program refinements through partial orders.