Tabbing is lost in .Net WebBrowserControler

We have a WinForms application running in .NET v2.0 which uses the built-in WebBrowserControl to view a simple web page.

The web page shows a login screen with text standard HTML text boxes.

When viewed in a standard web browser (IE6+, Chrome, Firefox, etc) the web page displays correctly and more importantly you can tab between the "Username" and "Password" textboxes.

The problem is when you view this web page from inside the application using the WebBrowserControl. It completely loses the ability to tab.

Any ideas why?