This example demonstrates the "client zoom" functionality, where OpenLayers stretches the layer div when the resolution is not supported by that layer's tile service.
The map of this example is configured with 22 resolutions, while the OSM tile server supports the first 19 resolutions only. When the zoom level is 19, 20 or 21 "client zoom" is applied to the OSM layer, i.e. the OSM layer div is stretched as necessary. The map's initial zoom is 18. So if you zoom in using the zoom bar's "+" button you'll effectively trigger "client zoom".
For demonstration purpose the map of this example has
fractionalZoom set to true. So "client zoom" also
applies if you choose arbitrary zoom levels using the slider of the
zoom bar, or shift-drag boxes to zoom to arbitrary extents.
"client zoom" therefore allows continuous zooming for tiled layers.
Enabling "client zoom" on a layer is done by passing
serverResolutions to the layer constructor.
serverResolutions is the list of resolutions supported
by the tile service. See the clientzoom.js source.