[manigandham]

> downloading the same huge asset bundle into localStorage

This would happen anyway no matter what storage or cache is used since all data is cleared after private browsing session is over.

Breaking localstorage (with an over-quota error) is not the way to deal with this. Polyfills are just more crap in complexity and downloaded bytes to compensate for a browser's issues.

It would be much better to have a navigator.isPrivate flag enabled so apps can check the environment accurately, not guess based on whether certain APIs work or not. It's not about SLAs but supporting standards. Deleting client-side data is all that private browsing needs to do.