XSLT matching PAGEID to an element ID - xslt

Question

How would I match two separate numbers in an XML document? There are multiple <PgIndexElementInfo> elements in my XML document, each representing a different navigation element, each with a unique <ID>. Later in the document a <PageID> specifies a number that sometimes matches an <ID> used above. How could I go about matching the <PageID> to the <ID> specified above?
<Element>
<Content>
<PgIndexElementInfo>
<ElementData>
<Items>
<PgIndexElementItem>
<ID>1455917</ID>
</PgIndexElementItem>
</Items>
</ElementData>
</PgIndexElementInfo>
</Content>
</Element>
<Element>
<Content>
<CustomElementInfo>
<PageID>1455917</PageID>
</CustomElementInfo>
</Content>
</Element>
EDIT:
I added the solution below to my code. The xsl:apply-templates that is present is used to recreate the nested lists that are lost between HTML and XML. What I now need to do is match the PageID to the ID of a <PgIndexElementItem> and add a CSS class to the <ul> it is a part of. I hope that makes sense.
<xsl:key name="kIDByValue" match="ID" use="."/>
<xsl:template match="PageID[key('kIDByValue',.)]">
<xsl:apply-templates select="//PgIndexElementItem[not(contains(Description, '.'))]" />
</xsl:template>
<xsl:template match="PgIndexElementItem">
<li>
<xsl:value-of select="Title"/>
<xsl:variable name="prefix" select="concat(Description, '.')"/>
<xsl:variable name="childOptions"
select="../PgIndexElementItem[starts-with(Description, $prefix)
and not(contains(substring-after(Description, $prefix), '.'))]"/>
<xsl:if test="$childOptions">
<ul>
<xsl:apply-templates select="$childOptions" />
</ul>
</xsl:if>
</li>
</xsl:template>

Answer 1

The XSLT way for dealing with cross references is with keys.
Matching: A rule matching every PageID element that it has been referenced by an ID element.
<xsl:key name="kIDByValue" match="ID" use="."/>
<xsl:template match="PageID[key('kIDByValue',.)]">
<!-- Template content -->
</xsl:template>
Selecting: A expression selecting every PageID element with specific value.
<xsl:key name="kPageIDByValue" match="PageID" use="."/>
<xsl:template match="ID">
<xsl:apply-templates select="key('kPageIDByValue',.)"/>
</xsl:template>