Using XSLT 2.0 with Saxon 8.9.0.3J I would like to use
xsl:message much more, but I don't know how to get
UTF-8 chars displayed correctly. Here is what I get when
I invoke test.xsl with saxon from the command line:

java net.sf.saxon.Transform dummy.xml test.xsl

a or ?that's the question  <---- I replaced a square by a true ?
<doc>a or d, that's the question</doc>>Exit code: 0

The square stands for x'E4 AC A0' (which would triggers base64
encoding in gmail by the way) and the text has not only lost the "d"
(x'C3 A4') but also the next 2 bytes ", " (x'C2 20').

test.xsl
<?xml version="1.0" encoding="UTF-8" ?>
<xsl:stylesheet version="2.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output omit-xml-declaration="yes"/>
<xsl:template match="/">
<doc>
 <xsl:message select="'a or d, that''s the question'"/>
 <xsl:value-of select="'a or d, that''s the question'"/>
</doc>
</xsl:template>
</xsl:stylesheet>