I think it would help to be designed to be usable by the keyboard alone but also to be usable by the combination of keyboard and mouse, and to work well either way, and you can do both together if you want to do (they are not different modes so you do not need to switch the mode).
Microsoft Windows has some of that; you can push ALT+SPACE and use the menu for move and resize by the arrows, but you can also use the mouse to move/size windows. Microsoft is not the best way, but this specific example is one of the things that can be done.
I also think that multiple document interface and tabbed windows could also be implemented in the window manager, that would not need to be the features of specific programs such as a web browser. (Some additional commands for application programs to interact with the window manager would be helpful; a window was created by another one, and is a part of the same program as this one, and some application commands are not specific to this window (but sometimes the user might want to start a separate instance for some windows anyways), etc.)
Microsoft Windows has some of that; you can push ALT+SPACE and use the menu for move and resize by the arrows, but you can also use the mouse to move/size windows. Microsoft is not the best way, but this specific example is one of the things that can be done.
I also think that multiple document interface and tabbed windows could also be implemented in the window manager, that would not need to be the features of specific programs such as a web browser. (Some additional commands for application programs to interact with the window manager would be helpful; a window was created by another one, and is a part of the same program as this one, and some application commands are not specific to this window (but sometimes the user might want to start a separate instance for some windows anyways), etc.)