If I were forced to pick one at gunpoint (because that’s feasible) I would choose images as being the biggest “yet to be solved” issue with responsive Web design. That’s a lot of air quotes and ridiculousness, but bear with me because I’m serious.
Images are almost a necessary evil. Yes we can make awesome stuff with CSS alone, and is already resolution independent. But more often than not we’re going to toss an image in there, and it’s not. Further, due to it’s bitmappy nature it doesn’t do us any favors in the bandwidth department.
A lot of really smart people have been working on this problem for a long time. To date, I’ve sat in the wings being impressed with both the proposed solutions as well as the research behind them. As much as everyone has tried, we still have yet to come up with a solution for this problem. Heck, we’re still thinking about the fundamental issue at hand. There are so many facets to this problem, every time you take a step closer to a solution, a whole new crop of issues may or may not spring up. It’s nuts.
If you’re not intrigued by the implications of this problem: have another cup of coffee, dude!