spec? localstorage errors are not standardized, every browser gives quota exceeded error different name and number, you have to use something like

'' e.name.toLowerCase().includes("quota")

But localStorage.setItem() throwing an error definitely is in the spec, so if someone isn't handling Safari's localStorage behaviour in Private Browsing Mode (that is, throwing exceptions) then they aren't handling the parts of the spec.

The web isn't built on specs, it's built on conventions. Safari breaks the convention.

the convention is that localStorage throws exceptions. it does in every browser. if a site breaks because it doesn't do error handling, then that's the site's problem.

You and many of the commenters miss an important point: Any apparent behaviour difference between private and non-private mode should be considered a bug, because sites can and do abuse it to detect private mode.

I disagree. You might be able to use it to _guess_ but all browsers can throw a quota exceeded exception under normal behaviour. I see this every now and then on mobile devices where the phone is so full the browser refuses to store more localStorage items.

This is exactly what I want to prevent though - I want to prevent dodgy sites from using the quota exceeded exceptions from localStorage to (100% accurately in my experience) guess that I'm browsing in private mode.