https://learn.microsoft.com/en-us/dotnet/api/system.web.ui.webcontrols.webcontrol.cssclass?view=netframework-4.8.1&redirectedfrom=MSDN
WebControl.CssClass Property (System.Web.UI.WebControls) | Microsoft Learn
Gets or sets the Cascading Style Sheet (CSS) class rendered by the Web server control on the client.
property systemweb uiwebcontrolmicrosoftlearn