Web browsers are designed to check the validity of SSL/TLS certificates when connecting to a secure website (one that uses HTTPS). If a certificate has expired, this means it is no longer valid, and the browser will typically display a warning to the user, indicating that the site may not be secure. The user would then have to choose whether to proceed at their own risk or not. This check is important because it helps to prevent man-in-the-middle attacks and ensures that the website is indeed operated by the entity that was originally verified by the certificate authority.