I always insist of developing the markup, in fact semantic markup, before it is styled. The basic reason is that content is still the king and the markup structures the content. Markup is what the search engines see. It is important enough to design it separately without thinking about how it looks. Then implement the layout using CSS. Then apply styles. This also helps in physically separating the three aspects – markup, layout and style. Do no change the markup to ever define how it looks. If you want a visual representation, check out a design lifecycle as it should be.