From
Visa Merchant Card Acceptance Guidelines --
The unembossed Visa card (e.g., prepaid card) may look and feel different, but it is a valid card that
can be accepted at any Visa merchant location that has an electronic terminal. Unlike an embossed
Visa card with raised numbers, letters, and symbols, the unembossed card has a smooth, flat surface.
From a merchant perspective, the processing of an unembossed card at the point-of-sale should
be seamless. There’s no need for new software, special hardware, or modified terminal procedures. You
simply swipe, insert, or wave the unembossed card just as you would an embossed card, then wait for
an authorization and obtain the cardholder’s signature.
edit @
danpeake -- This guide applies to all Visa-branded cards, including debit. Note also that a "prepaid card" (debit) is used as an example of an unembossed card.