Looking at your main.css file, there are three elements being styled: the html element, its child body element, and the element having the id of viewDiv (indicated by the pound sign). Each element has four property settings applied to it. The height and width settings specify that the element should take up all of the space of its parent container. The padding and margin settings specify that there should be no white space inserted on the inside or outside of the element, respectively. Basically, these styles produce a map filling the entire browser window.