Photoshop completely obscures the base reality of the medium of hypertext -- the document -- and directs all the focus onto surface concerns. I find that if you start by building pages like it's 1994 (but with div+span & class+id), and thinking about how to logically structure your document, the CSS can slowly fall into place iteratively.

Photoshop has all sorts of clever tools for pixel-perfection, and it's very easy to go overboard. The biggest problem is the easy delusion that the mockup is the object being designed, and the subsequent divorce from reality.

That's why I believe that if you're going to do anything outside a text-editor/browser/inspector, it should be in an unreal medium: dry-erase, crayons, cut-out construction paper, post-its, watercolors, FINGERPAINTS!