Similar to how Disney used a Multiplane Camera for Snow White, Multiplane Portals use stacked iFrames.
Sketch of a computer-controlled, 4-plane Multiplane camera sparetimelabs 
> The most famous multiplane camera was developed by William Garity for the Walt Disney Studios to be used in the production of Snow White and the Seven Dwarfs. The camera was completed in early 1937 and tested in a Silly Symphony called The Old Mill, which won the 1937 Academy Award for Animated Short Film. wikipedia
Portals are transparent “holes” in the top iFrames. The viewer can see through to the iFrames beneath. medium [⇒ Portals]
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DOT FROM two-level-diagram
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Multiplane Portals for the Metaverse medium