When Safari encounters a character not available in the current font family, say Ὦ, it attempts to find another installed font family that has the character and uses that. It seems most, if not all, OS X applications have this property.
When Internet Explorer on Windows encounters a character not available in the current font family, it just gives up and displays a square. At first I thought this might be Windows in general, but Firefox on Windows has the same behaviour as Safari on OS X.
So it's just IE.
I'd be interested if anyone other than IE users on Windows gets a square here: Ὦ