IE8 css selector

To target elements only in IE browsers i'll use

IE6:

* html #nav li ul {
    left: -39px !important;
    border: 1px solid red;
}

IE7:

*+html #nav li ul  {
    left: -39px! important;
}

Does anyone know how to target IE8?

I'm not going to get in a debate about whether or not this method should be used, but this will let you set specific css attributes for IE8-9 only (note: it is not a selector, so a bit different than what you asked):

Use '\0/' after each css declaration, so:

#nav li ul  {
  left: -39px\0/ !important;
}

And to build off another answer, you can do this to assign variou styles to IE6, IE7, and IE8:

#nav li ul  {
   *left: -7px    !important; /* IE 7 (IE6 also uses this, so put it first) */
   _left: -6px    !important; /* IE 6 */
    left: -8px\0/ !important; /* IE 8-9 */
}

source: http://dimox.net/personal-css-hacks-for-ie6-ie7-ie8/

2013 update: IE10+ no longer supports conditional comments.

Original answer:

Some people seem to be confused because this does not answer the letter of the question, only the spirit - so for clarification:

There is no such thing as a browser selector. There are hacks that take advantage of bugs and/or glitches in specific browsers' CSS parsers, but relying on these are setting yourself up for failure. There is a standard, accepted way to deal with this:

Use conditional comments to target IE only.

Example:

<!--[if gte IE 8]>
<style>
(your style here)
</style>
<![endif]-->

Everything inside the two <!--> will be ignored by all non-IE browsers as a comment, and IE versions that are less than IE8 will skip it. Only IE8 and greater will process it. 2013 update: IE10+ will also ignore it as a comment.