I totally agree with ideas expressed in "Overall model" section, but I can't see how "A proposal" section reflects them. What is said in "Proposal" is just a description of a mechanism of purging caches and other uncritical data. Both questions raised in "Model" section (one of ensuring reliable data storage and one of calculating sizes of IndexedDB data) are not answered, though, in my opinion, these are much more important problems than allowing webapp to help cleaning caches.