CSS trick for IE

I've just learned a very simple CSS trick that allows you assigning different attribute values if the page is rendered by IE. All you need to do is use '#' and '_' prefixes:

.myTestClass{
   width: 5px;  /* value used by all other browsers */
   #width: 7px; /* value used by IE */
   _width: 9px; /* value used by IE6 and older */
}

I don't think it's a part of the CSS official specification but it works ;)