What ITU-T standard defines public-key certificate formats widely used with TLS? The answer is X.509.
X.509 is an ITU-T standard that defines formats for public-key certificates and related certificate infrastructure. In TLS, an X.509 certificate binds an identity such as a domain name to a public key and is normally signed by a certificate authority.
A browser or other TLS client validates the certificate’s signature, checks its validity period and hostname, and follows a chain through intermediate certificates toward a trusted root. This process helps establish that the public key used during a secure connection is associated with the intended site.
X.509 is broader than HTTPS. The standard also covers certificate revocation mechanisms and certification-path validation, and X.509 certificates can support digital signatures and other applications. The distractors are different standards: E.164 covers telephone numbering, H.264 covers video compression, and X.25 concerns packet-switched networking.