button {
	_behavior: expression(( function(element) { 
		 if(typeof(behaviorOnMouseEnter) == 'undefined' ) { 
			 behaviorOnMouseEnter = function(element) { 
				 Element.addClassName(this, "hover");
			};
			behaviorOnMouseLeave = function (element) {
				Element.removeClassName(this, "hover");
			};
		}
		element.onmouseenter = behaviorOnMouseEnter;		
		element.onmouseleave = behaviorOnMouseLeave;
	})(this));
}