This would intend to be the opposite of the already-existing Email card. The email card allows content to be inserted on an email-only basis, but it would be useful to have the reverse!
It would be great to have a Website Card (or insert better name here).
“Only visible on the web, this card will not be published in an email.”
This would allow for circumstances where you might want to promote your newsletter or invite people sign up, without it bothering the folks that are already signed up!