The check with `xmllint` fails during validation. See log output below for more clarity
❯ xmllint '/Users/paria/0ad/binaries/data/mods/public/gui/common/global.xml' --relaxng '/Users/paria/0ad/binaries/data/mods/mod/gui/gui.rng' --noout
/Users/paria/0ad/binaries/data/mods/public/gui/common/global.xml:3: warning: failed to load external entity "/Users/paria/0ad/binaries/data/mods/public/gui/gui.dtd"
<!DOCTYPE objects SYSTEM "../gui.dtd">
Remove redundant references to deleted .dtd (Document Type Definition) files.
> //A DTD defines the structure and the legal elements and attributes of an XML document.//
Source: [[ https://www.w3schools.com/xml/xml_dtd_intro.asp | w3schools.com ]]
- `/art/textures/terrain/types/terrains.dtd` does not exist since rP12306 and was last changed at rP6391
- [[ https://code.wildfiregames.com/source/0ad/browse/ps/trunk/binaries/data/mods/public/art/textures/terrain/types/terrains.dtd | terrains.dtd ]]
- `/gui/gui.dtd` does not exist since rP15377 and was last changed at rP14098.
- [[ https://code.wildfiregames.com/source/0ad/browse/ps/trunk/binaries/data/mods/public/gui/gui.dtd | gui.dtd ]]
The problem is partially mentioned in #5556.