GNOME Wallpaper List XML Format Specification ("gnome-wp-list.dtd")

The way GNOME handles desktop wallpapers is with XML files following gnome-wp-list.dtd. There are basic tutorials and examples at the following two places, among others:

Using templates and examples, I was able to put together a working wallpaper slideshow, but I would prefer being able to do so programmatically.

I searched on both the open internet and the GNOME Gitlab but couldn’t find a copy of the DTD or any other proper specification. (Gitlab doesn’t offer site-wide source searching, though.) It does not appear that gnome-wp-list.dtd is a file on my system, but for all I know it’s buried in a GResource file somewhere.

Does anybody know where I could find a proper specification for the GNOME Wallpaper List XML format? Thanks!