If I want to develop fluid type calculations that adapt to local context, I’ll use em and cqi (container inline size) values. If I want my calculations to remain consistent across the entire page, I’ll use rem and vi (viewport inline size) calculations. In either case, I’ll define those values on body or other elements – so that 1rem always refers to the result of our initial negotiation, and doesn’t take on more complex meaning.

For spacing on the y-axis, the line-height (lh) unit is great or the rlh (root line-height). The x-axis can be handled with the vi or cqi units.

The right units for any situation are the ones that express most clearly what we mean – and sometimes what we mean requires a combination of units.

There’s no best unit, no best layout mode, and no best selector. When we use the entire language, we have more tools for clearly expressing our goals.

See Poetic CSS Architecture