Checkers that validate without transmitting

Checking something sensitive on a random website is usually a bad idea — these five exist so it doesn't have to be. The contrast checker scores text and background pairs against real WCAG thresholds as you pick. The password strength test estimates crack time locally; the password never leaves the field. The IBAN checker validates checksum and country format on your device, the card checker runs the Luhn algorithm for test numbers, and the readability checker grades your text and shows what drags it down.

All five run entirely in your browser, and never transmit what you check.