Adding styling information can change completely the look of a page. To see this in action, first have a look on the page displayed in the other window. Then, remove all four characters <!-- indicating the beginning of a comment on line 5 below and see how the appearance of the displayed page changes.

Most often, instead of putting all the styling information in a style declaration inside the document's head, one writes all the styling information in a separate file (known as a style sheet) which is then linked in the document. This way, the same style can easily be reused on many pages, and changes can be made globally to an entire site by only focusing on that style sheet.