Can a CSS selector reference another selectors property?

I added an answer to the question you linked: https://stackoverflow.com/a/11071806/137626

• You can use the same declaration block with as many selectors as you want by grouping them (selectors are separated by commas)
• You can't reuse the same declaration block later with a different CSS selector without rewriting the whole declaration block preceded by this selector or using a preprocessor/macro that'll do that for you. Or add it to the existing declaration block as above

Now with your example of an element resized by CSS itself: you could use CSS3 Media Queries and its @media rules containing as many declaration blocks as you want. These media queries would adapt to the width of viewport here.
Mixing expanding elements via animation and media queries that would've the reverse effect will be very soon very complicated (and I'll wonder what content you're playing with); if you want to Keep It Simple, then JS is the way to go. There are variables, loops, events ;) and you can start a CSS3 animation by adding or removing a single class from an element (or whatever CSS selector).
CSS3 won't replace JS (and you shouldn't use JS to style HTML as JS isn't activated or existing everywhere and there's already a nice fallback named CSS).