Developing for the modern web has a lot of challenges. Some has been overcome and addressed by the evolution of HTML, and there are lots still being actively discussed on the WHAT-WG mailing list, and more and more being implemented by the various browser vendors.
In fact it has grown way beyond just the evolution of HTML and is now an all encompassing goliath, we lovingly call The Open Web Platform. Then there is of course the device ecosystem. Growing and expanding at a dizzying pace. With larger form factors, devices with high density displays, audio and video capabilities that puts many DSLR cameras to shame.