@thomasguillot could you jot down your findings on the a11y aspects of this plugin, for posterity?

When the pop-up is visible. The keyboard navigation should only loop within the pop-up, until the dismiss (or submit...) button is clicked.

The timed pop-up should be displayed after the header but before the content.

It looks like amp-lightbox component does not trap focus, so this can only be achieved in a non-AMP way.

For that, the focus-trap library can be used. Here's a POC: #298

This hasn't changed according to recent AMP activity. I think we should put this issue on hold until we can replace amp-access with a unified way to check whether a prompt should be displayed (e.g. Bento).