Progressive enhancement is the practice of making a website more resilient in case of unsupported or missing technologies in the browser. Responsive web design is the practice of making the content dynamically respond to changes in the dimensions of its container (viewport), abstracting away the detail of the actual device capabilities.
Both have their place, but when applying the one, we shouldn't be thinking of the other. Seeing responsive web design as an enhancement to our sites is dangerous. We can often improve the layout on various devices, but this has the hidden cost of inserting more tricks in our code, making it less flexible, instead of more responsive. Choosing the right tweakpoints now doesn't mean that they will be right after two years, when new devices appear on the market and the popularity of the current ones changes. Responsive web design would mean that we find a way to adjust these points based on the context and not just manually insert media queries here and there. The goal of responsive web design is to make our websites future-proof, but it makes assumptions about how this future will look like. When we look at the past, we'll see that we are quite bad at predictions.
The work of web designers is dependent on the work of device makers and even the way they advertise their products. If we consider display characteristics like quality and size and ask ourselves which is generally more important for people, they would probably choose quality every time. A big display with low quality would always drive people away. It becomes then important for designers that seek quality and not only adaptation to different screen sizes to look at how device makers define this quality and to seek for the reasons why they don't do it another way. Why don't they just say “our device is 124% larger than this one” or “our pixel size is .09em smaller than on this comparison display”? Why do we continue to make high-resolution, pixel-powered displays, even if they are small, even in 2013? The small size was already there before Apple multiplied the pixels of the retina display. As long as displays remain rectangular and widescreen (which resembles the combined form of our eyes and the horizontal distance between them), it makes sense that the smallest unit that determines the quality of detail is also rectangular.
The resolution—often a main selling point,—is nothing else than multiplying the pixels in the horizontal and vertical dimensions of the screen to arrive at a fixed number. Screen size isn't responsive, only screen resolution can be adjusted, just to see that less than the native number of pixels shows inferior results. And in the past we tried to design for these different resolutions, with floats and percentage-based widths, speaking about “flexible” design. At the end we failed, because we were trying to control too many factors beyond our control. This is something great designers rarely do. I see the same trend now with responsive web design—we get excited about screen size, simply because we are at the start of redefining mobile devices. And we try to carefully control our tweakpoints, use vendor prefixes, polyfills and other normalization mechanisms, adding things without necessarily adding more value, without seeing how fragile our house of cards is. We are again trying to control too much from the beginning and I fear where this might end.
I'm not saying that responsive web design is bad. The idea behind it seems great to me, but this idea can't on itself compensate for what I feel is a wrong execution today. When it comes to progressive enhancement, I think that it works great and is much needed on most web design projects. (This will require the cost of additional planning for it too.) It's almost a blessing, because it can be achieved in a more “native” and intuitive way, which is why I think it can be more sustainable. Indirectly talking about “responsive enhancement”, the way we do it today, seems problematic to me. Before we put something on our sites, we must be always fully aware of its implications and side effects. Maybe we'll get to the point when responsive web design will be a real enhancement, but until then I'll try to think more of a “responsible enhancement”, one that will remain grounded on a stable base and will allow me to build a higher structure, despite taking more time. The Pyramid of Giza and the other Wonders of the World are a proof that great design can be considered design when being of high quality, not necessarily when widely spread. Maybe we should reconsider how far we spread our designs.