You would first need to target the operating system which can be best accomplished by using javascript such as:
if(navigator.userAgent.indexOf('Mac') > 0) {
//code
}
Now in order to target the browser version you can use things such as if ie triggers for internet explorer
<!--[if (gt IE 6)&(lte IE 7)]>
<link href="/css/IE.css" rel="stylesheet" type="text/css">
<![endif]-->
or webkit for mozila or safari
@media screen and (-webkit-min-device-pixel-ratio:0) {
your css here
}
final code to target internet explorer on mac would look something like this:
if(navigator.userAgent.indexOf('Mac') > 0) {
<!--[if (gt IE 6)&(lte IE 7)]>
<link href="/css/IE.css" rel="stylesheet" type="text/css">
<![endif]-->
}