XML processing with XSLT. This refers to the exercises described here. Using XMLStarlet or xsltproc, the XSLT stylesheets can be run like this:
$ xmlstarlet tr exercise-1.xsl ../xml/example.xml
$ xsltproc exercise-1.xsl ../xml/example.xml
In case you have problems understanding the identity transform used in various solutions, Wikipedia has an article about it.
There is obviously no XSLT solution of exercise 6 -- XSLT is not intended for XML validation.