Here's some basic Three.js support for Graphics3d objects. It is modeled on the corresponding function in SageMathCell: simply use threejs() to wrap any combination of points, lines and surfaces. This will save an HTML file with the JavaScript needed to render the scene and open it in a browser.
There are a great many features that can be added in the future, but I first would like to see how much interest there is in this. There have been problems surfacing with Jmol (#20978 for example) and I certainly think it would be preferable to have a native viewer that can be updated more quickly. The HTML format is also more portable than Jmol files, since it only requires a modern browser to run.
This version relies on an external CDN to retrieve the Three.js library, since it does not currently download to the hard drive during the build process. I'll need help on figuring out how to change that at some point, since I'm sure people will want to be able to view saved files offline.
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