Question

For example

#div {
   _height: 300px;
}

What versions of IE will take this?

Answer 1

The underscore hack will only work in IE6. There was a variation of it that worked in IE7:

#div {
   *height: 300px;
}

But neither of these are recommended as they produce invalid CSS. It would probably be a far better idea to have separate CSS files and include them conditionally for different browsers:

<!--[if IE 6]>
    <link rel="stylesheet" type="text/css" href="ie6.css" />
<![endif]-->

For more information on conditional comments, see this article.