What is inside the parenthesis? Why are there two sizes? This format is used to resize a SVG image

#logo {
	width: max(100px, 18vw);

It basically just picks the largest of the two values. Which only really makes sense if one of them is dynamic (like a % or viewport size).

So it will use whatever 18vw computes to as a value until it is equal to or less than 100px at which point it will be locked at 100px as the minimum value allowed.