Not too sure if this is an oddity of XSLT or of my parser (Xalan in cocoon) but I found that if the xsl:attribute value is not defined as the first thing, it is ignored completely. Eg

....
<img>
<xsl:attribute name="src"><xsl:value-of select="some-value"/></xsl:attribute>
<height>12</height>
</img>

will work

<img>
<height>12</height>
<xsl:attribute name="src"><xsl:value-of select="some-value"/></xsl:attribute>
</img>

will not.

This may seem obvious to some people, but it actually wasted a lot of time for us.