Amazon had a hierarchical configuration system for loading config that could override other config (or add to it, depending on the implementation). The Java framework would provide an endpoint to show you the final resolved config along with which file and line that value came from. Very helpful when trying to figure out why config was different than expected.

At least the inspector in Safari (and maybe other WebKit browsers) does something similar for CSS.

I’m all for it. It’s a pain when writing config systems (no longer just key/value, but key+value+meta), but very helpful. It can be a pain for things like JSON where libraries don’t give you that type of diagnostic information easily, however.