Cross-Origin iFrame

Full Title or Meme

The Inline Frame, or iFrame was introduced to allow isolated web pages from unrelated entities to embed content seamlessly into a web page.

Frames and Framesets were introduced early in browser history to enable refreshing only a portion of a web page to improve responsiveness of web pages in the days of low bandwidth data communications.
Identity features like OpenID Connect and WebAuthn 2 depends on the Cross-Origin iFrame for a seamless User Experience when identity is provided by a different web site than the Realying Party.

Iframes as a Security Feature does actually acknowledge some of the security problems with iFrames.