openlayers/examples/image-load-events.css

.map {
  background: #E0ECED;
}
.wrapper {
  position: relative;
}
#progress {
  position: absolute;
  bottom: 0;
  left: 0;
  height: 2px;
  background: rgba(0, 60, 136, 0.4);
  width: 0;
  transition: width 250ms;
}