"greater than" works but "less than" doesn't?

There are lots of XSLT processors you can download and install, like Saxon 6.5.5 for XSLT 1.0 (http://saxon.sourceforge.net/#F6.5.5) or Saxon 9.4 (http://saxon.sourceforge.net/#F9.4HE) for XSLT 2.0 or like AltovaXML (http://www.altova.com/altovaxml.html) for XSLT 1.0 or 2.0. Additionally some OS come with an XSLT 1.0 processor (e.g. xsltproc on LINUX I think, Windows with MSXML). I suggest you try and use one of them to run and test your XSLT code.

The http://www.w3schools.com/xsl/tryxslt.asp?xmlfile=cdcatalog&xsltfile=cdcatalog_choose is basically broken, it does not properly work with less than signs < as it basically reads out your entered code from a HTML textarea, then puts it into another one to then pass that content to an XML parser, however forgetting to properly escape the < sign twice (as that would be necessary with < being an escape mechanism for both HTML and XML).

I had the same issue, but instead of painful escaping and ruining the "prettiness" of the code, it was a lot easier to replace <xsl:when test="$value <= 10"> with <xsl:when test="not($value > 10)">