According to Photoshop tutorial, “the kerning and tracking values in Photoshop are based on 1/1000 em”. Text tracking affects spacing between characters, not words. So the CSS counterpart to setting text tracking to -2 would be letter-spacing: 0.002em
.
However, in most browsers, this won’t have any effect, since 0.002em
is such a small quantity. You may see a difference of one pixel if your font size is hundreds of points. On Firefox, an effect may be seen if the setting is applied to a long string; Firefox presumably internally computes the positions of characters using fractions of pixels, so even a very small letter spacing value can have a cumulative effect.
In general, you should not expect the details of text rendering to be the same across browsers, or the same in a a browser as in Photoshop.