Assuming the <article> elements are within an <articles> wrapper element, you can do this in XSLT with the rule:
<xsl:template match="articles">
<xsl:for-each select="article">
<xsl:sort select="title"/>
<xsl:copy-of select="."/>
</xsl:for-each>
</xsl:template>
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference