ABC uses ASCII characters for that - you write ^F for F sharp and _B for B flat.
None of "GUEST, Jon"'s characters works in my browser, they must be Windows-specific codes.
Here is an HTML rendering of what they look like to me:
¢¤ ♩
ÅÙ ♪
? ♫
¢ð ♬
ÅÛ ♭
? ♮
ÅÚ ♯