XML parser accesses Internet, why?

Why would the Java XML parser under Saxon8.7.1J need to access the
internet? The XML parser gave errors after I installed a new version
of Norton and Norton changed the internet settings so the XML parser
couldn't get to the internet. I fixed the Norton settings for Java
and then the errors went away. I'm parsing XHTML files with Saxon8.7.1J.

Error on line 3 column 62 of file:/C:/aa-xml/hltapi/MLD.ori.html:
   SXXP0003: Error reported by XML parser: Cannot read from
   http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd (www.w3.org)

Thanks,
Mark