MarkerCluster.css

.leaflet-cluster-anim .leaflet-marker-icon, .leaflet-cluster-anim .leaflet-marker-shadow {
	-webkit-transition: -webkit-transform 0.3s ease-out, opacity 0.3s ease-in;
	-moz-transition: -moz-transform 0.3s ease-out, opacity 0.3s ease-in;
	-o-transition: -o-transform 0.3s ease-out, opacity 0.3s ease-in;
	transition: transform 0.3s ease-out, opacity 0.3s ease-in;
}

.leaflet-cluster-spider-leg {
	/* stroke-dashoffset (duration and function) should match with leaflet-marker-icon transform in order to track it exactly */
	-webkit-transition: -webkit-stroke-dashoffset 0.3s ease-out, -webkit-stroke-opacity 0.3s ease-in;
	-moz-transition: -moz-stroke-dashoffset 0.3s ease-out, -moz-stroke-opacity 0.3s ease-in;
	-o-transition: -o-stroke-dashoffset 0.3s ease-out, -o-stroke-opacity 0.3s ease-in;
	transition: stroke-dashoffset 0.3s ease-out, stroke-opacity 0.3s ease-in;
}