But when will <xsl:template match="@href"> be selected?

A template is only executed if its match attribute matches a node in the
current node list (and has highest priority/import precedence)

Since attributes are not in the default selection of apply-templates
this template will only fire if you explictly include the attribute axis
in some apply-templates. But I thought you said that you were not
selecting attributes when you got the error.

As it uses xsl:attribute, then it should generate a runtime error if it
is not immediately after a new element node has been started in the
result tree. But if templates are not applied to attributes, then this
can never happen.

You can try it out with Xalan.

Not without a full (small) example stylesheet and source doc, I can't.

<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
   <xsl:template match="root">
       <xsl:attribute name="test">test</xsl:attribute>
   </xsl:template>
</xsl:stylesheet>

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