Validation warnings to show:
1. Gadget with type=styles having non-CSS files
2. Packaged gadgets not having at least one JS file (after T198758)
3. PeerNon-packaged gadgets not behaving styles-only gadgetsJSON files (after T198758)
4. Style-only gadgets having peers
5. Peer gadgets not being styles-only gadgets
6. Scripts/styles/datas not being of JS/CSS/JSON contentmodel, respectively, or not existing altogether
7. Invalid page actions specified
8. respectivelyNon-existent namespace numbers specified (after https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Gadgets/+/624517/)
Currently all of these validations do take place at some point, but no warnings are shown to the user. The problematic parts in gadget definitions are silently ignored.
**Approaches**
1. Expand GadgetDefinitionValidator (for gadget definition namespace only) – Pre-save hook that blocks the save in case of issues. The existence/contentmodel of the resource files could change, so it may not be a good idea to validate those here.
2. Show warnings in Special:Gadgets – These can cover all kinds of validations.