Shortly saying, DTD is a little old and limited format for XML file descriptions. Relax-NG is simpler and more powerful. Most importantly, it allows elements to appear in any order while DTD needs hacks for that. It can also put further restrictions on data types. If the idea is accepted, I can prepare necessary patches to repoman and the Relax-NG schema.
isn't this an EAPI change ?
(In reply to comment #1) > isn't this an EAPI change ? Hm, hard to say. We can't bind this to a specific EAPI as metadata.xml is independent of single ebuilds in the package dir. PMS denotes only the metadata.dtd location. If we add Relax-NG schema as an additional spec to metadata, leaving DTD intact, I guess PMS would be fine as-is. Relax-NG could effectively impose further restrictions on our metadata while keeping it compliant with the DTD.