Lowercase letters, numbers and hyphens only — this is the unique id of your mod.
Official schema — schemaVersion 1. The exported zip contains judge_pack.json plus an assets/ folder with your images and sounds. Only PNG, JPG, JPEG or WebP images (recommended: transparent 512×768 or larger) and short MP3 sounds are allowed. No HTML, CSS, JavaScript, SVG, fonts, external URLs or absolute paths.