We are entering the post-pc era of the Internet. All web sites need to be mobile friendly. Muse needs to facilitate the creation of at least three sizes of web pages without the designer having to build three seperate sets of code. We need desktop size, tablet size and smart-phone sizes. The design needs have the ability to be smart enough to select elements which fit each of these screen sizes. The design needs to be able to drop some elements based on screen size. Things like that.