SDOC is a format for 3D scenes on the web. Just like HDOC and CDOC it is a static format, meaning that it cannot include any scripts.
SDOC is currently not defined, but it will be similar to CDOC in that the main content will probably be located in a section that will use some popular format for 3D scenes, just like CDOC uses a popular 2D vector graphics format (SVG).
SDOC will probably have <head> section, <copy-info> section, and <connections> section just like HDOC, and CDOC.
SDOCs may contain HDOCs, CDOCs and even other SDOCs. All those documents may be included by reference.
What will 3D scenes be used for?
One use case I can think of is having something like a site map. Only instead of links to different pages you could have a 3D scene where you can surround your reader with your content.
In the video example, I move things around. This will be possible with local scenes that user creates. SDOCs downloaded from the web will probably be immutable.
Also, in the demo the 3D scene looks basic. But since the SDOC format will be based on some popular 3D format, you should be able to create scenes of any complexity.
