Gaps in the current permissions model

[miketaylr]

We have the following issues which point out some gaps in the current model:

#178: describes "implicit" permissions (i.e., permissions are granted by virue of a user interactive with a "chooser" UI, such as <input type=file>)
#189: describes "conditional" permissions (i.e., permission may be conditioned upon user activation or site engagement).
#250: describes "grantable" permissions that may be granted by the UA when requested, but denied otherwise.