@href is what tells xsl:import and xsl:include which module to pull in.
Without one the processor refuses the stylesheet (XTSE0010), and an empty one
names the stylesheet itself, which is refused too. An empty href is usually a
module moved or a path deleted mid-edit. Write the path of the module meant.
Incorrect:
<xsl:import/>
<xsl:include href=""/>
Correct:
<xsl:import href="common/attributes.xsl"/>
<xsl:include href="common/tables.xsl"/>
The XSLT 3.0 spelling _href is checked the same way.