Web Control Hierarchy
From Xojo Documentation
The built-in web controls have an inheritance hierarchy.
Object is the base class for all webpages, dialogs and controls.
WebControl is a subclass of Object and is the base class for all web controls including those with and without a user interface.
WebUIControl is a subclass of WebControl and is the base class for things that can contain other controls including WebPage, WebDialog, WebContainer, WebPagePanel (and WebTabPanel because it's a subclass of WebPagePanel) and WebRectangle.
UserGuide:Web Pages topic