Validate an RadioDNS (ETSI TS 102 818) SI.xml file offline using xmllint

combined.xsd

<?xml version="1.0" encoding="UTF-8"?>
<schema elementFormDefault="qualified" xmlns="http://www.w3.org/2001/XMLSchema">
  <import namespace="http://www.w3.org/XML/1998/namespace" schemaLocation="xml.xsd"/>
  <import namespace="http://www.worlddab.org/schemas/spi/33" schemaLocation="spi_33.xsd"/>
</schema>

validate.sh

#!/bin/sh

wget -nc -q -O ./xml.xsd http://www.w3.org/2001/xml.xsd
wget -nc -q -O ./spi_33.xsd https://www.worlddab.org/schemas/spi/33/spi_33.xsd

xmllint --nonet --noout --schema combined.xsd ./SI.xml

There is a warning that a schema has already been imported:

spi_33.xsd:8: element import: Schemas parser warning : Element '{http://www.w3.org/2001/XMLSchema}import': Skipping import of schema located at 'http://www.w3.org/2001/xml.xsd' for the namespace 'http://www.w3.org/XML/1998/namespace', since this namespace was already imported with the schema located at 'xml.xsd'.

This can be hidden by adding --nowarning but that might hide other useful warnings too.