"convert" apparently expects to find an ATI processing instruction in
xsl stylesheets before it will execute HTML transformations. Otherwise
it only produces XML.
The PI should take the following form and be inserted as follows:
<?xml version="1.0"?><?APT StylesheetID Title="FOOBAR" CompositionTypes="htmlfile"?><xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">