html>body #header {margin-bottom:1em}

#header {
  padding: 2em; 
  border: 0.5em; 
  width: 15em; 
  voice-family: "\"}\""; /* IE5 cannot see from here */
  voice-family:inherit; width: 10em
}

/* commented backslash hack v2 \*/
#isnotMacIE5 { display: block; background-color: #060; color: #fff; }
#isMacIE5 { display: none; }
/* end hack */

<style type="text/css">@import url(cssfile.css);</style>

/*/*/.emptyrule{}
div {width:553px;}
/* */

div {
width: 100px; /* Seen by OP5 */
}
div {
\width: 140px; /* Seen by IE5.x only */
w\idth: 100px; /* Seen by NS6, IE6, OP6, Mozilla */
}

Note: If Nav4 sees even one escape anywhere in the CSS, it will discard the entire sheet. So it is vital that this hack be hidden from that browser, by means of @import,

/* the following rules apply only to IE6 and below */
* html{
}
 
* html body{
}
 
* html .foo{
 
}

/* the following rules apply only to IE7 */
*+html .foo{
}