This example demonstrates the "client zoom" functionality, where OpenLayers stretches the layer div if the current resolution isn't supported by that layer's tile server.
This map is configured with 22 resolutions, while the OSM tile server supports only the first 19 resolutions. When the zoom level is 19, 20 or 22 "client zoom" is applied on 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 see that "client zoom" is applied.
The map is even configured with 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.
Enabling "client zoom" on a layer is done through the
serverResolutions option. See the clientzoom.js source
to see how this is done.