This is 100% browser and OS specific, we do not override the default browser text box handling of these keys. You can experiment here:
Take http://codepen.io/anon/pen/uryJl which contains a plain html text box.
In different web browsers, type in
test line one
test line two
test line three
And then try ctrl+z.
Depends on the browser and whether you naturally pause or not when typing. Play with it and see.