Ok… but the following topics in your list have nothing to do with an API specification, which is what they say their goals is on the Goals section:

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“A few UI differences between chrome's extension popup and firefox's one means you'll need to potentially leave out features for one browser. - UX differences between browsers means you'll need to write extra code, and maybe a few extra tutorials. - Huge huge difference between publishing on chrome vs on firefox - Safari requires xcode, and therefore macos to publish”

Bottom line is I don’t think there is going to be an api standard around “distribution and publishing areas”