The Popover API (https://chromestatus.com/feature/5463833265045504) specifies the behavior for two values of the `popover` attribute: `auto` and `manual`. This feature describes a third value, `popover=hint`. Hints, which are most often associated with "tooltip" type behaviors, have slightly different behaviors. Primarily, the difference is that `hint`s are subordinate to `auto`s when opening nested stacks of popovers. So it is possible to open an unrelated `hint` popover while an existing stack of `auto` popovers stays open. The canonical example is that a <select> picker is open (popover=auto) and a hover-triggered tooltip (popover=hint) is shown. That action does not close the <select> picker.
One common use case, "Hints" or "Tooltips" was not covered by the original Popover API proposal. This use case was originally included in the prototype, but because some technical issues couldn't be worked out in time, this part of the feature was removed from the initial shipping feature. This intent adds back support for Hints/Tooltips.
Explainers: https://open-ui.org/components/popover-hint.research.explainer