CSS is essential for page layouts. In the past, you might have used tables, <center> and <font> tags. With the addition of CSS, all of this formatting is taken out of the markup itself and put into a separate CSS script. This allows for easier creation of content with a more consistent look and feel.
There are several parts to the YUI CSS library. The first that needs to be included in this project is the Reset stylesheet. Every browser has a slightly different way of formatting CSS elements; the Reset stylesheet attempts to normalize these differences so you get a consistent look across all browsers.
The next part is the Grids stylesheet. This stylesheet is responsible for layouts, giving you both liquid and center-column options. By using the Grids stylesheet, you can build layouts easily and ensure proper display across all web browsers.
The final two parts are the Base and Fonts stylesheets. Base provides some miscellaneous things that will be used later on as you build your blog and Fonts provides consistent cross-browser front sizes and families.