Although I agree that modern web apps suffers from the size already and additional 300kb doesn't hurt in this case, but one of the most popular use cases for the client DB is the offline capability for an small footprint web app. You know, it is when your connection is poor or even absent. 300kb hurt really badly.
> Also, couldn't the implementation of that storage spec. be shipped in the standard JS web APIs (i.e. by the browser)? Why would it be in every app?
Actually, that is exactly what happens to WebSQL and IndexedDB. WebSQL got deprecated, they were not able integrate SQLite, and created IndexedDB, which is hated by many developers. Just an example:
https://news.ycombinator.com/item?id=27511941
> Also, couldn't the implementation of that storage spec. be shipped in the standard JS web APIs (i.e. by the browser)? Why would it be in every app?
Actually, that is exactly what happens to WebSQL and IndexedDB. WebSQL got deprecated, they were not able integrate SQLite, and created IndexedDB, which is hated by many developers. Just an example: https://news.ycombinator.com/item?id=27511941