Some versions of the XSLT stylesheets don't handle the missing slash
correctly (they concatenate directory and file name without the slash).
This might never have worked correctly.
<!-- Parameters -->
-<xsl:param name="base.dir" select="'html'"></xsl:param>
+<xsl:param name="base.dir" select="'html/'"></xsl:param>
<xsl:param name="html.stylesheet" select="'stylesheet.css'"></xsl:param>
<xsl:param name="use.id.as.filename" select="'1'"></xsl:param>
<xsl:param name="make.valid.html" select="1"></xsl:param>