Bring developers in while ideas can still change
Developers belong in the design process from discovery, before concepts harden and well before client sign-off. Early involvement gives a creative team a clear view of technical feasibility while there is still room to shape the idea. A developer can spot implementation constraints, identify edge cases and suggest possibilities that make an experience stronger, not simply smaller. That can mean checking colour contrast, planning for mouse, touch and keyboard inputs, or making sure a design works across different devices. A finished visual is a poor moment to discover that a key interaction cannot be built as intended. Designer-developer collaboration works best as a shared problem-solving process, with both disciplines able to influence the work before approval.
Sources: Designing and developing web experiences for everyone, No creator is an island