[lexicality]

Sorry, I completely forgot about the existing semantic tree view element that exists and can be interacted with visually but doesn't provide any accessibility or keyboard support because the browser manufacturers decided to skip that one.

Or are you talking about a situation where the developer has implemented a custom component (aka "done something") which doesn't use the native focus system and therefore requires additional work to make accessible?

[cluckindan]

If by ”done something” you mean the devs made custom widgets have the proper ARIA roles so they’re usable for people who use a keyboard to navigate, or who need screen readers and their plethora of different navigation modes. This is usually the case when a suitable standard component does not exist or is not well supported across browsers. Hierarchical tri-state checkboxes come to mind.

The native focus system typically works just fine, but JS is needed for keyboard interactions and to set things like aria-activedescendant.