Why doesn't min() (or max()) work with unitless 0?

Solution 1:

You need to add a unit to 0 otherwise it's confusing for the browser to handle the comparison between a uniteless value (a <number>) and a value with unit (a <length>) and the top property accept a <length> not a <number>

top: max(0px, 120vh - 271px)

To understand this, you need to follow the specification:

The min() or max() functions contain one or more comma-separated calculations, and represent the smallest (most negative) or largest (most positive) of them, respectively.

Then for calculations:

A calc() function contains a single calculation which is a sequence of values interspersed with operators, and possibly grouped by parentheses (matching the <calc-sum> grammar),

So the content of min()/max() is treated like the one of calc() then from the type checking

A math function can be many possible types, such as <length>, <number>, etc., depending on the calculations it contains, as defined below. It can be used anywhere a value of that type is allowed.

and

Note: Altho there are a few properties in which a bare <number> becomes a <length> at used-value time (specifically, line-height and tab-size), <number>s never become "length-like" in calc(). They always stay as <number>s.

You may get surprised but using top:0 is valid while top:calc(0) is not. To make the latter valid it needs to be top:calc(0px). Same logic for min()/max()

Worth to note that the same also apply to clamp() since it's equiavalent to max(MIN, min(VAL, MAX))

Related: Why doesn't css-calc() work when using 0 inside the equation?