The HtmlTextWriter class implementation of the IsValidFormAttribute method returns true for all attributes. Override the IsValidFormAttribute to limit the attributes that can be rendered in the opening tag of a <form> element.
Notes to Inheritors
Override the IsValidFormAttribute method to return true only for those attributes that can be included in the <form> element of the markup for which the class derived from the HtmlTextWriter class is intended.
Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)