To avoid needing to customize tons of templates, I do the following for that situation. This allows us to add role="highlight" to any element and have it be highlighted yellow. This needs to be in a customization layer that imports the docbook xsls because it relies on doing an xsl:apply-imports:

For html (assuming you have something like .remark{ background: yellow; } in your css):

<xsl:template match="text()[ ancestor::*/@role = 'highlight' ] | xref[ ancestor::*/@role = 'highlight' ]" priority="10"><span class="remark"><xsl:apply-imports/></span></xsl:template>

And for fo:

<xsl:template match="text()[ ancestor::*/@role = 'highlight' ] | xref[ ancestor::*/@role = 'highlight' ]" priority="10"> <fo:inline xmlns:fo="http://www.w3.org/1999/XSL/Format" background-color="yellow"><xsl:apply-imports/></fo:inline> </xsl:template>

David