This post is here to save your time during GUI development in Mathematica. And one way to do this is to know where limits are and to be aware of features that are awaiting.
Usually I don't have to care (it hurts my brain though) about misaligned details, additional/missing pixels. But currently I'm participating in a project where the design matters and I have to follow examples made by a graphics designer. I have to workaround 10 "features" per hour, and I'm not talking about fancy stuff.
This is intended to be a cheat-sheet for GUI development, based on our experience.
What is on topic here
Links to guidelines making GUI creation process more stable, predictive.
Links to guidelines on how to achieve robust dynamic performance of GUI-s.
Links to known bugs and workarounds for GUI-related issues, preferably posted as separate Q&A threads.
Tips about communication with designer/manager about requirements.
People are used to the fact that you can create very nice layout with css in no time. How to explain that it is impossible, or not worth the time spent, in Mathematica? How to explain why getting rid of "white line" in some secondary menu took me half a day, and I've failed?