Why isn't this SVG? Font icons are only applicable when there is actually an appropriate corresponding Unicode character for the icon, as text is supposed to be selectable/copyable, and display fine (albeit in a different typeface) if I turn off web fonts and have full Unicode coverage. Some of the icons that Github implements are in Unicode, yet they don't even use the characters for those.
For an example of when it's appropriate to use a custom font for an icon, I use a custom font for U+1F50D Left-Pointing Magnifying Glass in the search button on my website. Github uses U+F22E [INVALID] instead for their search icon.
Every major current generation browser and last generation browser supports SVG images except IE 8. So IE 8 users just wouldn't see the icons - not a big loss since I suspect IE 8 users make up a tiny amount of the user base.
For an example of when it's appropriate to use a custom font for an icon, I use a custom font for U+1F50D Left-Pointing Magnifying Glass in the search button on my website. Github uses U+F22E [INVALID] instead for their search icon.