Currently non eTLD+1 frames get do not have any storage, which means frames expecting normal acting localStorage, document.cookie, etc will have web compat problems.

non eTLD+1 frames should instead get

1. Empty storage (e.g. the frame at start doesn't see any stored values)
2. site length storage (all storage is deleted when all top-level frames pointing at the site are closed / deleted)
3. API compatible storage (e.g. document.cookie acts in the weird ways document.cookie acts, etc.)
4. Isolation from other ways of getting to the domain's normal storage (e.g. either prevent them from spinning up workers, or ensure that those workers get similar ephemeral / isolated storage)
5. partitioned-by-frame storage (two A frames under the same B frame see the same storage, but A frames under different B frames see different storage, A under B sees different storage than A under C or top-level-A)
6. (possibly) Storage Access API escape valve (sites wanting more storage can use storage access API to get global / unpartitioned storage)