The same-origin policy became the browser's core isolation rule, letting users visit mutually untrusted sites while restricting one site's scripts from reading another site's private state.
OAuth emerged from a practical web problem: let one application act on a user's behalf at another service without forcing the user to hand over the password for that other service.
Netscape's SSL and the IETF's TLS turned encryption, authentication, and integrity into a reusable transport layer for web traffic, creating the technical basis for HTTPS and secure network applications.