Rather than thinking about [[css]] as a bunch of unrelated properties, think of them as connected layout algorithms.
Properties apply differently in the context of different layout modes. The most common layout modes are:
Layout modes also have its own variants, e.g. the 'Positioned' mode has variants for 'relative', 'absolute', etc.
Given a property like z-index, this will actually have no effect in Flow layout, but will do something in other layout modes. Instead of thinking that z-index needs other properties to make it work, shift the mental model to z-index only works in particular modes.
Another example is that something as ubiquitous as width works differently in Flow and Flexbox. Flexbox only uses it as a suggestion, whereas Flow considers it a rule. It's not a quirk of Flexbox, it's a deliberate implementation difference.
Properties require the context of the layout mode to work. In order to understand CSS, consider the layout mode you're workng in.