re: #5: At my previous job we wrote our own UI toolkit with React and css-modules. We had many dev-only views that we used to inspect and test our components. It worked great because it let you visualize all the states a component could have for a wide range of inputs, so you knew they were all being handled well. For example: If you get a long input prop do any of your styles break from visibility overflowing? If a nullable value is missing, are you styling it appropriately while showing a placeholder? Unfortunately it wasn't open source, but it's being done.