In the last twenty years, the evolution of web systems has been driven along three dimensions: the processes used to develop, evolve, maintain and re-engineer the systems themselves; the end products (the pages, content and links) of such processes; and finally the people dimension, with the extraordinary shift in how developers and users shape, interact and maintain the code and content that they put online. This paper reviews the questions that each of these dimensions has addressed in the past, and indicates which ones will need to be addressed in the future, in order for web system evolution to be sustainable. We show that the study on websites evolution has shifted from server- to client-side, focusing on better technologies and processes, and that the users becoming creators of content open several open questions, in particular the issue of credibility of the content created and the sustainability of such resources in the long term.