When activated, the extension disables borders, outlines, background colors, text colors, box shadows and text shadows.
Seeing a web page in such a rudimentary form helps you evaluate the information hierarchy of your design. Suddenly it becomes much easier to make sure you are adhering to a coherent visual grid. Without the visual clutter it allows you to see the typographic structure a lot easier.
Source code (licensed under GPLv3): https://github.com/JannesMeyer/chrome-black-and-white/