The maturation of web design, simplification of engineering, and sophistication of users' tastes has created an environment where designers are strongly incentivized to not just learn the basics of HTML and CSS but become full-fledged front-end engineers. It's not hard to go from there to coding entire apps on your own and moving into the back-end.
For me, it's not necessarily about incentivising my designers, but more my designers understanding what's possible, and more to the point what's impossible, when designing. If they can hand over their designs to our front-end developers knowing how they'd write the CSS, there'd be no going back and forth saying this won't work. And if we're really under the pump they can chip in with some code.
Hard thing is finding designers who are into both design and code.