Stands for Cascading Style Sheets. CSS gives web designers great control over the typography of HTML documents. Before CSS, there is no way to specify font sizes in points or pixels, specify line spacing, define different link colors, etc. But what makes CSS even more attractive is that you can define an external CSS file holding all the style definitions, and all files on your web site can use this file. When you need to make a change in say, level 1 headlines, you only modify that external CSS file, and your whole site adopts the change immediately.