How does the OS X font fallback system work?
OS X normally will not display a character in the Private Use Area unless you manually set the font in the app.  I understand this is intentional, since every font could have a different glyph at the same PUA code point, and it only makes sense to display something when the user specifies the font.

I think it should also work when the right font is selected via CSS or other markup in a document being read.

Some earlier versions of OS X did not have this "feature" and did display PUA stuff.  I don't know how they chose which font to use when more than one had something at the code point selected.