Currently the developer guide deals mostly with guidelines for laying out code, and how to submit patches. We should have some guidelines on how to deal with basic communication elements of the UX. The following is a list I can think of off the top of my head, I'm sure there are others: 1) Idempotent functions, and how we report NOPs of these. 2) Text of add/remove buttons (i.e 'Add <entity>' vs 'Add'). 3) Reporting failures/success in the WebUI and client.
We have such a guide now, which we can expand as needed when we find anything missing. https://beaker-project.org/dev/guide/ui-guidelines.html