[VE][html5] Add Subject Here Getting an error for using Unicode PUAs!

Error [html5]: ""
NOTE: Whenever possible, give the address of the document you were checking.


Hello,

I'm getting the following error:  *Document uses the Unicode Private Use
Area(s), which should not be used in publicly exchanged documents. (Charmod
C073)*

Why shouldn't Unicode PUA be used? What's wrong with them??
I'm using font-based icons from IcoMoon ( http://icomoon.io/app/ ).
Please advise.

Thank you
Anon SU