Feature request: Icon fallback

[got...@gmail.com]

Hello, I realized when an icon it's not defined due a typo or because has been removed, it quite hard to notice that the icon is missing. Would be great to have some kind of fallback icon, so by default for any icon-* the :before element has warning text, and then be overriden by the :before element of the correct icon.
What do you think?

Regards

[Keyamoon]

That would require adding another selector like [class^="icon-"]:before, [class*=" icon-"]:before, which might have some bad performance implications.