To target Internet Explorer 6 and Internet Explorer 7, start your properties with \\*:

.hide-on-ie6-and-ie7 {
    *display : none; // This line is processed only on IE6 and IE7
}

Non-alphanumeric prefixes (other than hyphens and underscores) are ignored in IE6 and IE7, so this hack works for any unprefixed property: value pair.