WebAssembly only gives better performance for things that shouldn't be done in the UI main-thread. As long as you don't do number crunching in the mouse move event handler, I doubt WebAssembly can beat vanilla JavaScript for reactive UIs.
Once you work with the DOM, the (small) overhead of calling from WASM into a JS shim or any performance difference between WASM and JS doesn't matter. The internal DOM implementation will dominate the performance profile unless you do really dumb things.
