The wheel event is in uievents spec here:
https://w3c.github.io/uievents/#event-type-wheel


Though I am not sure where in uievents defines the extension to Element.idl interface. garykac@ do you know what we should link to here?

Since these events fire on elements and bubble, the right thing would be to add it to GlobalEventHandlers:

partial interface GlobalEventHandlers {
  attribute EventHandler onmousewheel;
};

https://w3c.github.io/selection-api/#extensions-to-globaleventhandlers does this.