Jen Simmons’s suggestion is spot-on: it just got a lot more important to design from a sensible, small screen-friendly document structure built atop semantic HTML.
I'm less excited about WebKit coming to watchOS than I am about the overall impact it will have. The more we focus on developing experiences that can be consumed at any size on any fidelity device, the better the web will become.
Nice post from Erik Runyon about some of the technical specifics as well.