Try this instead...
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="html"/>
<xsl:template match="/">
<html>
<STYLE>
TR.clsOdd { background-Color: #feeded; }
TR.clsEven { background-color: #beefee; }
</STYLE>
<head>
</head>
<body>
<p>
<center>
<table>
<tr>
<th>Client Number</th>
<th>Client Name</th>
<th>Client SSN</th>
<th>Relationship Description</th>
</tr>
<xsl:apply-templates select="Data"/>
</table>
</center>
</p>
</body>
</html>
</xsl:template>
<xsl:template match="data1">
<tr>
<xsl:choose>
<xsl:when test="position() mod 2 = 1">
<xsl:attribute name="class">clsOdd</xsl:attribute>
</xsl:when>
<xsl:otherwise>
<xsl:attribute name="class">clsEven</xsl:attribute>
</xsl:otherwise>
</xsl:choose>
<td><xsl:value-of select="./info1/clnum"/></td>
<td><xsl:value-of select="./info1/clname"/></td>
<td><xsl:value-of select="./info1/ssn"/></td>
<td><xsl:value-of select="./info1/rel_desc"/></td>
</tr>
</xsl:template>
</xsl:stylesheet>
Hope that helps,
John Pallister
XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list