You know the feeling. The mockup was alive. The shipped version is... a very respectful taxidermy of the mockup. Same layout, same colours, dead eyes. The overshoot on the menu, the stagger on the cards, the little exhale on page load — gone. Where did they go?
They got handed off, that's where. A handoff is a translation, and translation loses the poetry first. The spec carries the what — sizes, colours, spacing — but the how lives in a thousand tiny judgement calls. Is this ease-out eager or lazy? 60ms of delay, or 90? No document survives those questions. Only someone who cares can answer them, and by handoff time that someone has usually left the room.
So I ship my own code. Not because I don't trust engineers — every engineer I've worked with writes better code than me, and they know it. It's because the last 5% of craft can't be specified. It can only be performed. When the person tuning the curve is the person who dreamed it up, nothing gets lost in translation, because there is no translation.
And no, the advice isn't 'every designer must become an engineer' — relax. It's smaller than that: get close enough to the medium to finish your own sentences. Enough CSS to tune your own transitions. Enough JavaScript to prototype the thing you keep failing to explain in words. Not for the job market. For the work — so it arrives in production still breathing.