It may look like a crappy Electron app, but Figma has a quite interesting architecture. The browser editor is developed in C++ and cross-compiled to JavaScript with emscripten. The rendering engine looks like its handling HTML, but it's actually rendering their own document format for cross-browser consistency. They have their own CRDT implementation to handle multi-user edits.
[0] https://www.madebyevan.com/figma/building-a-professional-des...
[1] https://www.madebyevan.com/figma/
[2] https://www.madebyevan.com/figma/how-figmas-multiplayer-tech...