How to increase and decrease application font size in OS X

Wouldn't it be handy if, while running Mac OS X, you could enlarge some of the fonts and applications like Mail or Safari? Some of those fonts can be relatively small. All you need to do to increase font size in an OS X application is to press the command, shift and plus keys. To make a font smaller, in turn, all you need do is press the command, shift and minus keys. For more on using these key commands to obviate the need for squinting, watch this Apple tutorial.