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 two property settings applied to it. The height setting specifies that the element should take up the full height of its parent container. The margin setting specifies that there should be no white space inserted on the inside of the element. Basically, these styles produce a map filling the entire browser window.