The PanelDesigner.OnBehaviorAttached method calls the System.Web.UI.Design.ReadWriteControlDesigner.OnBehaviorAttached base method. Then the PanelDesigner.OnBehaviorAttached maps the System.Web.UI.WebControls.Panel.BackImageUrl and System.Web.UI.WebControls.Panel.HorizontalAlign properties to the appropriate style by using the PanelDesigner.MapPropertyToStyle(string, object) method, if these properties are defined.