I mean with Xib, you can e.g. specify how the window should behave when you are resizing it since you are specifying layout constraints as opposed to having to hardcode the layout. With CSS, your only recourse is media queries but those are a lot less fine-grained. Look into http://gridstylesheets.org (and more specifically http://gridstylesheets.org/guides/ccss/) which is kinda the love child of CSS and Xib (or the technology that inspired the layout technology).