Code that mediates between your application and the resources that it attempts to retrieve.

• Your parser asks for a DTD:

  http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd
  

• Or your application loads a schema:

  https://www.w3.org/2001/datatypes.xsd
  

• Or your application loads a stylesheet:

  https://cdn.docbook.org/release/xsltng/current/xslt/docbook.xsl