Just think of XSL as a vocabulary you can use to create nice PDFs.
In theory, you can use XSL in a browser...
The nice thing about XSL is that you can:
Use XSLT to re-arrange the output.
Specify page sequences for different page layouts.
Control the page layout (body, headers, footers, etc.).
There is an expression language that lets you calculate expressions against the rendered tree.