data: <data attr="1" />

xsl:

    <?xml version="1.0" encoding="UTF-8"?>
    <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:xs="http://www.w3.org/2001/XMLSchema" exclude-result-prefixes="xs" version="2.0">

      <xsl:template match="/data">
        <one>
          <xsl:value-of select="@attr = (2,3,4)" />
        </one>
        <two>
          <xsl:value-of select="(@doesntexist,2)[1]" />
        </two>
      </xsl:template>
    </xsl:stylesheet>