This bug was found with Sablotron 0.60. When a stylesheet is using:

...
<xsl:output method="xml" version="1.0" encoding="UTF-8" omit-xml-declaration="no" />
...

The resulting XML will have the following declaration:

<?xml version="1.0" encoding="UTF-8">
...

Notice that according to XML 1.0 the declaration should end with '?>', not just '>'. So the '?' is missing, hence the resulting XML document is not well-formed.

Thanks.