Right now, debugging non-trivial flexbox layouts can involve an immense amount of trial and error. Tooling to improve that situation would be wonderful. I suspect this would be some sort of cross-cutting CSS / layout tool, possibly doing things like exposing internal values for various internal calculations (eg how "auto" is calculated for some things) as visual cues. Something simple that I've been playing around with is at <https://gist.github.com/dmose/5f754f500a418fbabc6b>, which just colors elements according to their box model. I suspect with some real experimentation and design, one could come up with something fairly compelling.