Detecting whether content changes is still a Javascript thing, because CSS can't detect content changes via a pseudoclass or something like that.
CSS doesn't interact with the DOM that way, it's just for applying styles on elements. If the browser detects changes in the DOM, it will reapply the CSS where necessary, but the CSS doesn't 'know' that the DOM (and it's contents) has changed.
So you need Javascript to detect this and you could then either directly insert CSS via .css() or add a class and do the animation from there, but you probably knew that already ;)