Then I agree completely, and must have misunderstood what was proposed by a handler. Typically a handler will launch an external application such as mailto, ftp, magnet, etc.

If we want to run code in browser there is WASM.

So is the proposal that it would it be beneficial to have a DOS-like OS or x86 emulator in WASM for running COM files?

Yes, that would be better and more sandboxed than dosbox running outside the browser.