master.xml

<data>
    <aspectratios>
        <item name="onebyone">1</item>
        <item name="onebytwo">0.5</item>
        <item name="onebythree">0.33333</item>
        <item name="twobyone">2</item>
        <item name="twobythree">0.66667</item>
        <item name="twobyfive">0.4</item>
        <item name="threebyone">3</item>
        <item name="threebytwo">1.5</item>
        <item name="threebyfive">0.6</item>
        <item name="fivebytwo">2.5</item>
        <item name="fivebythree">1.66667</item>
    </aspectratios>
</data>

master.xsl

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

  <xsl:output method="xml" indent="yes" />

  <xsl:template match="/">
    <xsl:call-template name="find-nearest-value">
      <xsl:with-param name="list" select="aspectratios"/>
    </xsl:call-template>
  </xsl:template>

<xsl:template name="find-nearest-value">

    <xsl:param name="value" select="0.8"/>
    <xsl:param name="list"/>

    <xsl:for-each select="list/item">
        <xsl:sort select="(. - $value) * not(0 > . - $value ) - (. - $value) * (0 > . - $value)"/>
        <xsl:if test="position() = 1">
            <xsl:value-of select="@name"/>
        </xsl:if>
    </xsl:for-each>
</xsl:template>


</xsl:stylesheet>