Checks if the resource pack has a namespace folder (e.g. assets/minecraft). Although it's not required to have a namespace folder, it's most likely what you want.
Checks if all json files located in a namespaces models directory are JsonObjects. This is a basic check, which should never fail, but it's a requirement for all other model based checks.
Checks if a model has at least one texture assigned to it. This check does not fail if the model has an existing parent model.
Checks if the texture files referenced in the model exists.
Checks if item overrides are correct and the referenced model exists.
Checks every file if it has been referenced in any json file. Most files need to be referenced somewhere, but this check may falsely detect files, which are in use. This will not flag for vanilla texture/model overrides. Files can be ignored with the ignore option using shell globs or regex.
Checks every PNG file, that will be loaded into the texture atlas, if it limits the mipmap levels. If the mip level drops below 4, the game displays certain textures with lower quality (e.g. leaves).
Checks every PNG file, that is located in any resource pack directory, if the texture is an actual image.
Checks if the texture file referenced in the font provider exists.
Checks if a font file defines a character multiple times, which overrides the character.
Checks if a model json, which is present in the default pack, is also present in a specified overlay. This Validator requires some additional configuration to run and may be used to make sure every model in a specified path is overridden in a specific overlay (e.g. to fix certain models in certain game versions).