How is Direct3D retained mode implemented in the browser?
d3drm has been implemented from scratch:
https://github.com/isledecomp/isle-portable/tree/master/mini...The entire project can be compiled targeting Emscripten. There's nothing particular to the browser implementation
The last update video I watched from MattKC indicated they were still deciding how to approach this. Based on the commit dates in that tree, it looks like they must have completed it.