Picture, for example, a card which has a small OLED display which displays an amount and a merchant name. You press a little button on the card, and an authorization is generated, cryptographically signed with an embedded key, and sent to the card reader (which also provides the power for the card).

Such a reader can be built into laptops, keyboards, smartphones, available as small stand-alone USB devices, etc.. Web browsers, POS systems, etc. can send a request to the reader and tell the user to place their card on it and check the card's display.

Transactions without a valid signature can simply be discarded.

If the system is implemented properly, the only way to commit fraud should be to physically steal the card.

(A more paranoid version could include buttons on the card for entering a passcode, so that even if the card is stolen, it would be difficult to use, at least before being reported stolen.)