This sounds like a mostly theoretical problem. Sure, it could be annoying, but if the quota simple is a little smarter (e.g. allowing a few filled subdomains rather than just 1, and/or always allowing the root subdomain to fill up, and/or evicting subdomains from localstorage on an LRU basis) then the chances of this happening non-maliciously are very small - and when push comes to shove, there's no guarrantee localStorage will persist anyhow, so every site needs to be robust in the face of data loss anyhow.