Thanks for the report. You found a real bug, and it's fixed in the next release: https://feedback.objo.dev/bug/1421
You were right that it wasn't implemented for web: ContainerControl is a desktop standard-library class, and the web target doesn't provide it. Studio's add menus were only checking whether an item type was allowed in the current build scope, not whether the owning project type could actually compile it, so a web project could be handed a template that fails with Unknown superclass 'ContainerControl'.
The add menus, the Insert menu, and the navigator's clipboard paste are now gated on the project type. In a Web project you'll only be offered what the web target can build (e.g. Class, Enumeration, Interface, Module, and Web Page). The same applies to command-line and worker projects, and to Shared code in solutions that contain any non-desktop project, since shared code compiles into every project. Desktop projects behave exactly as before, and Window, Menu Bar, and Toolbar are gated the same way.
For reusable sub-layouts in web projects today, GroupBox and TabPanel remain the container options. Adding a proper web ContainerControl is something I am likely to add in the future, but there's nothing to announce yet.