Stimulate

@media (min-width: 900px) {
  .floating {
        width: 100%;
    }
    .fixed {
        position: fixed;
        top: 45px;
        z-index: 99999;
    }
}

var new = document.createElement('div');
new.innerHTML = xhr.responseText;
var old = document.getElementById('mydiv');
var parent = old.parentNode;
parent.insertBefore(old, new);
old.remove();