This is a Web 2.0 Single Page Application / Rich Internet Application graphical menu system, and the size and shape of the tabs can be changed by editing the tab images. The size and shape of the "pages" can be made to match their contents by adjusting height and width parameters.
The "pages" that are formed by the content in the submenu <div> areas can be hard-coded in the appropriate parts of the main index.html page. Or they can be loaded with AJAX from files named, for example, subMenu11.txt.
The 'subMenu11.txt' file in this example is loaded with AJAX when the '1st Main Tab' tab is clicked and then the 'Submenu 1 Item Two' submenu selection is clicked. But note that this feature only works when the site is accessed at a remote location on the Internet, with the exception of Firefox which is able to load files with AJAX when 'localhost' is used.
The item number of the submenu that's been selected is added to the URL in the browser address line, and this enables pages to be reloaded at the same menu item. This feature also allows individual menu items / pages to be saved in the browser bookmarks / favorites folder.
The last submenu items that were selected within particular tabs are returned to when the tabs are clicked again.