HTML 5 local Storage size limit for sub domains

HTML5's localStorage databases are size-limited. The standard sizes are 5 or 10 MB per domain. A limit of 5 MB per origin is recommended.

The following is stated −

User agents should guard against sites storing data under their origin's other
affiliated sites, e.g. storing up to the limit in a1.example.com,a2.example.com,
a3.example.com, etc, circumventing the mainexample.com storage limit.

For size limit −

A mostly arbitrary limit of five megabytes per origin is suggested.
Implementation feedback is welcome and will be used to update this suggestion
in the future.