You have heard the word proportion in Architecture. If you want to call a Architecture classical, the key concept should be proportion, i.e, correlation of one element to another. Same in the case of UI. User Interface(UI) acts as a medium of interaction between the User and the Web. In a web application the most important aspect is the user interface, because this is the only part end users are going to view.
Before we start, I want to you to know how we reached to this article"Why is proportion important in UI".