Start with a named template and pull in the files or URIs with the Saxon
specific argument to the "collection" or "uri-collection" function:

<xsl:template name="xsl:initial-template">
 B  <xsl:value-of select="uri-collection('?select=*.xhtml') ! . || ' : '
|| doc-available(.)" separator="&#10;"/>
</xsl:template>

In theory I think that should check with doc-available if the file is
well-formed or not. Haven't tested however.

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  version="3.0"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  exclude-result-prefixes="#all"
  expand-text="yes">

  <xsl:template match="/" name="xsl:initial-template">
    <xsl:value-of select="uri-collection('?select=' || $search-pattern)
! (. || ' : '
|| doc-available(.))" separator="&#10;"/>
  </xsl:template>