Old but sometimes useful single character CSS hacks for targeting IE6 and IE7, typically when testing some bugfix for these old browsers.

ie6-and-ie7-one-character-hacks

#myelement {
    color: #999;  /* shows in all browsers */
    *color: #999; /* notice the * before the property - shows in IE7 and below */
    _color: #999; /* notice the _ before the property - shows in IE6 and below */
}