Why should line-height be unitless in CSS?

I've often heard that line-height should always be unitless. In my earlier coding years, I didn't question it much, but lately I've come to wonder why that is. In my mind 1.5 and 1.5em should produce the same result, right? Turns out, they don't.

There's a subtle difference between the two and it has to do with the fact that line-height is an inherited property. A unitless value will be inherited as-is, meaning the actual value will be recalculated for each element, accounting for the font-size of the element. However, a line-height with any unit will be calculated once and then inherited as a fixed value. This can cause vastly different results, especially if the declaration is in the body element or something similar.

Speaking of the body element, it could be a good idea to define your base line-height as a unitless value there to minimize repetition:

body {
  line-height: 1.5;
}

So, is line-height with units prohibited and should we always use unitless values? Not necessarily. Factors such as codebase conventions, design systems and personal preference play a role here. For example, maintaining an exact, perfect vertical rhythm with unitless line-height values can be a bit tricky. In such cases, using line-height with units can be a good idea, but remember to be consistent to avoid headaches.