[hackingthenews]

I wonder how broad the definition of effectiveness is in their implementation. The visual method might have limitations, considering that many CSS properties are only "effective" when you start moving/interacting with elements, e.g. resizing the window/elements, or hovering over elements or using other cursor actions.

[jchw]

Looks like good ol’ screen diffing. It may be possible to use heuristics to detect when a rule might be important for layout in other configurations but this is probably good enough for the stated purpose of highlighting what CSS is important for learning.