The explainer has a section dedicated to the behaviors of each of these popup types, but there are some gaps:

1. Should "nested" popup=hint (tooltips) be allowed, in the same way that popup=popup is allowed to nest? It seems like tooltips and other popup=hint UI should really be constrained to be exactly "one at a time", with nested tooltips prohibited. But are there use cases for tooltips nested within tooltips?
2. Should popup=popup be allowed to "nest" inside tooltips? Again here, it seems like the answer should be no, but perhaps there are use cases?
3. When a tooltip is shown while there are open popups, and then the topmost popup is hidden (either with light dismiss or via hidePopup()), should the tooltip be hidden also? It seems like "yes", but what do UI folks think?
4. How should initiallyopen (maybe renamed to defaultopen soon) behave when there are all three popup types. It would seem that only the first occurence of either <div popup=popup initiallyopen> or <div popup=hint initiallyopen> should be shown upon page load. It would also seem that an unlimited number of <div popup=async initiallyopen> should be auto-shown on load, independent of popup=popup or popup=hint.

I have a CL up to implement my best guesses at the above questions, so perhaps an easy way to get answers is for folks to wait for that to land in a Chromium Canary and test it out. But thoughts appreciated.