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).

I still don't understand why the issue is more complex for native mobile apps than it is for web mobile apps? Don't they all have to accommodate for the same screen sizes?

It's not that it's more complex, it's that you can do things with native layouts that you'd have to use JS to achieve on mobile.

Good thing browsers run JS, eh?

Programming is all about abstraction management. Doing layouts with JS is using the wrong abstractions. CSS was originally conceived for doing document layouts but it's less suitable for web-application layout. We can leave the discussion here, if the article I posted didn't convince you, nothing probably will.

Well, I'm somewhat still arguing in the context of the OP's assertion that "the hackery required to attempt to imitate native mobile apps is horrific". I not only disagree that it's horrific, but actually think it's pretty darn good. I agree with you that it's not perfect, but perfect is the enemy of good.

Are you talking about Auto Layout? You can write the same thing programmatically, in fact it's quite nice, though not as obvious as modifying frames.

I was. But you are still hooking into the same layout API as if you'd use IB.