I would just tell the client that the browser is 8 years old and get over it personally, If its good enough for yahoo (to not support it) its good enough for the rest of the world too.
googling "target ie5mac" gave me this stylesheet hack
- <style type="text/css">
-
\/*
-
@import("stylesheet.css");
-
*/
-
</style>
http://www.webmasterworld.com/forum83/2627.htm
Which is ugly, but should suit your purpose.
Alternatively you can target specific versions of IE but not mac/win with IE conditional comments.
My personal favourite.
Here are all the permutations I use, ripped straight from the textmate HTML bundle.
[HTML]<!--[if IE 5.0]> IE Conditional Comment: Internet Explorer 5.0 only <![endif]-->[/HTML]
[HTML]<!--[if IE 5.5]> IE Conditional Comment: Internet Explorer 5.5 only <![endif]-->[/HTML]
[HTML]<!--[if lt IE 6]> IE Conditional Comment: Internet Explorer 5.x <![endif]-->[/HTML]
[HTML]<!--[if lte IE 6]> IE Conditional Comment: Internet Explorer 6 and below <![endif]-->[/HTML]
[HTML]<!--[if IE 6]> IE Conditional Comment: Internet Explorer 6 only <![endif]-->[/HTML]
[HTML]<!--[if gte IE 7]> IE Conditional Comment: Internet Explorer 7 and above <![endif]-->[/HTML]
[HTML]<!--[if IE]> IE Conditional Comment: Internet Explorer <![endif]--> [/HTML]
[HTML]<!--[if !IE]><!--> IE Conditional Comment: NOT Internet Explorer <!-- <![endif]-->[/HTML]