CSS font-weight thicker than 900?

In CSS 3 there's another way to make the font size bolder:

color:#888888;    
text-shadow: 2px 0 #888888;
letter-spacing:2px;
font-weight:bold;

EDIT:

For some sort of weird reason this doesn't look as pretty as it did over an year ago. It only works with text-shadow of 1px (on most common fonts, other thicker fonts might still work with 2px). And with text-shadow of only 1px, there's no need for such a large letter-spacing.

color:#888888;    
text-shadow: 1px 0 #888888;
letter-spacing:1px;
font-weight:bold;

To add to Gogutz answer, you can go even bolder by stacking up the text-shadows in a grid. Comma separate each on the line:

.extra-bold {
  text-shadow: 0px 1px, 1px 0px, 1px 1px;
}

You can use text-shadow instead of font-weight

text-shadow: 0px 1px, 1px 0px, 1px 0px;