This page demonstrates how a single local font (in this case, Tahoma - shown in red) can be used to provide fallbacks for a wide variety of web fonts (shown in blue below).
This is accomplished by using size-adjust in conjunction with font metric overrides.
If you don't have Tahoma installed locally, this screenshot shows what this demo would look like if you did.
Demos
@font-face { font-family: fallback-a; src: local("Tahoma"); size-adjust: 100%; ascent-override: normal; descent-override: normal; linegap-override: normal; }