Dan Mall wrote a great piece about The Hot Potato design handoff process. That’s where he tackles the misconception that the design handoff should be from designer → engineer and only happen once. Dan argues ideas should be passed back and forth between designers and engineers all the time.
Also, in my experience, this is the best way to work across engineering and design. Once the hand-off happens a designer should expect to sort of work like a consultant, constantly giving feedback and making sure that all the edge cases have been covered; responsive design, accessibility, interactions, and animations, etc. Those things are especially hard to get right in the first mockup.
