Every interesting interface problem I've worked on lived in the space between the design file and the codebase — and in most organizations, that space is nobody's job.
The designer hands off a static frame. The engineer implements what the frame shows. And everything the frame cannot show — how the transition feels at 60fps versus 30, what happens in the 400 milliseconds while data loads, how the layout breathes between breakpoints, what the keyboard user experiences — gets decided by whoever happens to be typing, usually under deadline, usually without noticing a decision was made.
The gap has a shape
You can list what falls into it: motion, loading states, error states, focus order, scroll behavior, the feel of drag, the timing of everything. None of these are in the mockup. All of them are the product. A button is a rectangle in Figma; whether it feels solid or mushy is decided entirely in the gap.
Teams sense this, which is why handoff rituals keep growing — redlines, annotations, motion specs, hour-long walkthroughs. These help, but they're fundamentally lossy compression. The design intent is a living thing, and every artifact flattens it.
What design engineering actually is
The fix isn't better documents. It's a person who refuses to let the gap exist — someone who can open the design file and question the spacing system, then open the codebase and change the easing curve, and who considers both actions the same job.
The title doesn't matter. The refusal does.
I've seen this role called UX engineer, design technologist, creative developer, front-of-the-frontend. The common thread is ownership of the seam: when something feels wrong, the design engineer doesn't file a ticket across the wall — they fix it, because there is no wall.
How to hire for it
Don't screen for the union of two skill sets; you'll reject everyone honest. Screen for the instinct. Ask a candidate to critique a real interface, and listen for whether they move fluidly between why the hierarchy fails and why the scroll stutters. The ones who don't notice a difference between those two questions are the ones you want.