Notes:
- The setting of "xsi:schemaLocation" is ignored.
- The gmd namespace must be "http://www.isotc211.org/2005/gmd", e.g. xmlns:gmd="http://www.isotc211.org/2005/gmd".
- By itself, a tag without namespace (e.g. <MD_Metadata> instead of
<gmd:MD_Metadata>) is always valid because it is not associated with
any schema.