I don’t think Excel web will ever be running the calc engine browser side, no. The only way I could see this happen would be via compiling the core to wasm, which I don’t think is worth the engineering effort.

Excel has this legacy (but extremely powerful) core with very few people left that knows all of it. It has legacy bugs preserved for compatibility reasons as whole businesses are ran on spreadsheet that break if the bug is fixed (I’m not exaggerating). The view code for xldesktop is not layered particularly well either leading to a lot of dependencies on Win32 in xlshared (at least back then).

Is it doable? I’m sure. But the benefits are probably not worth the cost.

Thanks for the interesting info! Yeah, maybe Excel web will someday support local calculations via wasm, but for now I think I will stick with Excel desktop with add-ins.