XSLT knows nothing about HTML, you can use it to generate any HTML that you want (valid or invalid).
However, in standard XSLT 1.0, and in particular with client-side XSLT, there is a restriction that you can only output one document (i.e. one HTML page) from a single transformation.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference