When I change :

a {
    ...
    font-family:Helvetica;
    ...
}

The square style just change and I still don't see the icon.

I don't have this bug in Opera 26 and Opera 25.

The only difference with Opera 27 is the default font is Helvetica instead of Helvetica Neue but with Helvetica in Opera 27 I still have this bug ... Maybe it's an OS X problem.

Here some examples :

Example 1
Example 2
Example 3

Hi

Till the version 27, special font characters like arrow, or social network icon don't appear correctly, I have a square instead of the character ?

Screenshot

Is it normal ?