Example of a Bing Maps layer.
When the Bing Maps tile service doesn't have tiles for a given resolution and region it returns "placeholder" tiles indicating that. Zoom the map beyond level 19 to see the "placeholder" tiles. If you want OpenLayers to display stretched tiles in place of "placeholder" tiles beyond zoom level 19 then set maxZoom
to 19
in the options passed to ol.source.BingMaps
.
See the bing-maps.js source to see how this is done.