Margins and IE 4.5 for the Mac

This code in a stylesheet produces a 1 em bottom margin for h1 tags in the
associated HTML document under IE 5.0 for Windows 98:

h1 {
 margin: 0,0,1em,0;
}

However, the margin does not appear under IE 4.5 for the Macintosh. Any
ideas?

Thank you,
Jeff Ratliff