Promisified U2F API for Browsers

u2f-api is a client-side JavaScript library that provides a promisified interface for interacting with the Universal 2nd Factor (U2F) API in web browsers. It aims to abstract away browser-specific implementations of U2F, offering support for Chrome (including its historical extension methods), Opera, and Firefox 58+ (though with some caveats regarding multi-domain registrations). The library provides core functions such as `register()` and `sign()` for managing U2F security keys, alongside `isSupported()` and `ensureSupport()` for client capability detection. The current stable version is 1.2.1, released in January 2021. Its release cadence is infrequent, suggesting a maintenance-only status. A key differentiator is its modern Promise-based API and its efforts to normalize U2F interactions across different browsers, but it's crucial to understand that U2F itself is largely a legacy standard, with WebAuthn being the modern successor for FIDO authentication. This library requires a complementary server-side implementation (e.g., using the `u2f` npm package) to function fully.