I think with a lot of the QA, interactions and subtle design nuances, I'll hand folks handling the front-end the css/js for those interface components. No extra effort on their part, unless they have to worry about polyfills for browser support. The good thing is if you're providing the front end code, you've already fine tuned the details and don't have to worry about the nitty gritty details being off.
It's confusing we are talking about a UX/UI role here so it's a bit of a problem here. If it was pure UX I wouldn't expect him/her to hand actual components of code. Whose job is it to choose the best tools?
I think with a lot of the QA, interactions and subtle design nuances, I'll hand folks handling the front-end the css/js for those interface components. No extra effort on their part, unless they have to worry about polyfills for browser support. The good thing is if you're providing the front end code, you've already fine tuned the details and don't have to worry about the nitty gritty details being off.