<xsl:apply-templates select="document(@href)/id('content')"/>

use of id() depends on the parser reporting the dtd types, it's often
safer to use key. If you are using xslt2 you can use
select="key('id','content',document(@href))"
where the key is defined by
xsl:key name="key" match="*[@id]" use="@id"

David

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