Unlike CSS which simply applies styles directly to elements in a single pass through the document, XSL gives you the opportunity to do major reorganization of your document. This capability comes at the cost of simplicity. To ease the burden on developers, the designers of XSL have split the process into two parts: transformation and formatting.
The transformation alters the structure of the input document and adds presentational information in a hybrid format composed of formatting objects. A formatting object is a container for content that associates styles and rendering instructions with the content. It is compact and easy for a person to understand. The formatting objects are arranged in a tree that retains structure used in building the final presentation.
The result of this transformation is a temporary file that you feed to an XSL-FO formatter. Through a complex series of steps, the formatter calculates the final geometry and appearance of the presentation and churns out a file suitable for printing or viewing on a screen. When it's finished, the formatting objects are flushed from memory and you can discard the temporary FO file. It is important to understand that you are not meant to write your own XSL-FO markup. Make all of your stylistic corrections in the XSLT stylesheet and let the tools do the rest.