Keyboard Keys for Browsing and Other Functions:
Microsoft Windows® provides support for keyboards with special keys for browser functions, media functions, application launching, and power management. The WM_APPCOMMAND supports the extra keyboard keys. In addition, the ShellProc function is modified to support the extra keyboard keys.
 
It is unlikely that a child window in a component application will be able to directly implement commands for these extra keyboard keys. So when one of these keys is pressed, DefWindowProc will send a WM_APPCOMMAND message to a window. DefWindowProc will also bubble the WM_APPCOMMAND message to its parent window. This is similar to the way context menus are invoked with the right mouse button, which is that DefWindowProc sends a WM_CONTEXTMENU message on a right button click, and bubbles it to its parent. Additionally, if DefWindowProc receives a WM_APPCOMMAND message for a top-level window, it will call a shell hook with code HSHELL_APPCOMMAND.
 
Windows also supports the Microsoft IntelliMouse® Explorer, which is a mouse with five buttons. The two extra buttons support forward and backward browser navigation. For more information, see XBUTTONs.