Interactive email
An email you can act inside — open a menu, pick a size, answer a poll.
Interactive email moves a step of the journey into the message itself. It comes in two kinds: tricks built from ordinary HTML and CSS, and AMP for email, which is a real format with real scripting.
The CSS kind works by exploiting hover and checkbox behaviour. Where it is unsupported nothing happens at all, so the email must still make sense as a plain one — that fallback is the whole design problem.
It is worth the effort when it removes a step people abandon, and not worth it as decoration.