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.