I think the explainer would benefit from a document/section that explains the core differences to sandboxed iframes like the following:

<iframe src=bloburl sandbox="allow-popups allow-top-navigation-by-user-activation" />

Sandboxed frames are treated as a top level origin if they're opaque (!allow-same-origin).

• My understanding is the core difference here (in opaque-ads) is that the frame has an origin but is isolated from the embedding frame.
• https://github.com/WICG/fenced-frame/blob/master/explainer/opaque_src.md#proposed-solution seems like a non serializable URL object that once loaded has a serializable URL inside the frame.

I'm quite interested if the existing primitives of the web can be used rather than these new ones.