If it's a pixel-perfect screenshot, getting the CSS right is actually extremely easy as long as there's surrounding text. It's easy to recognize a font, easy to figure out its size, and easy to figure out the coordinates. No more than a couple minutes of trial and error.

It's only difficult if it's been resized+compressed lossily, if it's a photograph of a screen, etc. And since font rendering can be different between Windows and Mac, you might have to try it on each one for a perfect match.