Guest View is a mechanism for embedding a page inside another page. Unlike iframes, the embedded content has a distinct FrameTree. See the FrameTree explainer for details.
Guest View is the core implementation of several embedding features implemented in the extensions/ layer. Notable examples include:
Guest Views are currently implemented using inner WebContents. By end of 2023, we plan to migrate Guest Views to use MPArch like other features which nest FrameTrees.
For debugging <webview>, the Browser Sample [source] app serves as a basic demo.