A string containing the markup used to render the System.Web.UI.WebControls.BaseValidator at design time.
If the System.Web.UI.WebControls.BaseValidator.ErrorMessage or System.Web.UI.WebControls.BaseValidator.Text property of the associated control that is derived from the System.Web.UI.WebControls.BaseValidator class is an empty string (""), or if the System.Web.UI.WebControls.BaseValidator.Display property is set to the System.Web.UI.WebControls.ValidatorDisplay.None field, the BaseValidatorDesigner.GetDesignTimeHtml method sets the System.Web.UI.WebControls.BaseValidator.ErrorMessage property to the control ID, which is enclosed in brackets ([]) and sets the System.Web.UI.WebControls.BaseValidator.Display property to the System.Web.UI.WebControls.ValidatorDisplay.Static field. The BaseValidatorDesigner.GetDesignTimeHtml then calls the System.Web.UI.Design.ControlDesigner.GetDesignTimeHtml base method to generate the markup, and restores the control properties to their original values, if necessary.