Programming (and especially programming for the web) boils down to reducing the number of “unknowns” on a project to a manageable number. One is fine, two is a stretch, and three is asking for trouble. … One of the recurring discussions we have at our studio is: “Should our designers learn markup and start doing some of this design in the browser?” We’ve heard a lot of persuasive arguments for it, but in the end, we decided that the main focus should be how to get our designs into the browser earlier in the process, instead of who should be doing that work.

http://alistapart.com/article/prototyping-your-workflow