If there is a UI designer in the team then he is responsible to hand off the assets to the UI developer in high fidelity converted from the low fidelity provided by the UX guy. If there is only a single guy doing the UI/UX, then he is going to do the job on his own.

Moreover, a UI developer needs assets to build a layout, if the product is already out and there is only a small update that needs to be rolled out. In that case, the UI developer can just ask for the low fidelity ideation flow concept from the UX guy and can then resuse the elements set earlier in the product styleguides and templates to develop that particular update. ( Assuming UI developer has an average understanding of design elements ).

For just HTML layouts, sometimes a bit of information about the styleguides, typography and colors with low fidelity work flow prototype from the UX guy can do the job.

Probably things like the user flow, demographics, user stories, actions that need to be performed by users. From there the UI designer should make decisions on shape, sizes, proportion, typography, and color that will suit the objectives clarified by the UX team.