A planner is not necessarily a builder, themer, developer, or coder. A planner isn't always the project manager either. So, what does a planner need to know to support development?

Assumptions

If we assume that the role of planner has been instrumental in gathering requirements for and defining the design of the site, then we can assume the planner is the person who has a complete understanding of what needs to be developed. Or at least has access to those who can clarify the requirements and design if questions come up.