Particularly good combination of scope (including differences across systems and browsers) and depth (including both gotchas and workarounds).
I didn't even know about isTrusted until a couple of weeks ago when I was writing some widgets to speed up some tasks by automating a bunch of web controls. I couldn't get my auto-paste to work, discovered that flag and went down a rabbit hole of trying to over-ride a safety mechanism on my own local browser (not easy!).