line-height is a css property
lead [rhyming with red]:
Originally a strip of soft metal used for vertical spacing between lines of type. Now meaning the vertical distance from the baseline of one line to the baseline of the next. Also called leading.
When you declare line-height in css you don't need to declare a unit.
The line-height property defaults to being relative to the font-size. So a unitless line-height is equivalent to declaring a percentage i.e:
or an em
Since you don't need either. I say just leave it off. But whatever you do, don't use pixels.
Regardless of whether or not you are writing a book, or making a website - you generally only need 3 or 4 values for line-height.
When a line-height has a 1:1 ratio to the size of the type (line-height: 1) this is known as setting type solid. Most typefaces set at solid are unreadable - so this should be avoided in most cases.
One reason to set line-height to 1 is for single lines of text that need to be set flush at the top with a corresponding element that has embedded media or a solid background. For example a piece of text next to an image where we want the top of the typographic element to be perfectly aligned at the top with the image.
line-height: 1.2 to 1.3
Titles require less leading than body copy. You generally want something around 1.2-1.3.
line-height: 1.5 to 1.75
Many people with cognitive disabilities have trouble tracking lines of text when a block of text is single spaced. Providing spacing between 1.5 to 2 allows them to start a new line more easily once they have finished the previous one. - w3
I generally set copy to 1.5 to aid in readibility. But this becomes a matter of taste between this defined range and is not generally an issue of readability.
Often times you see line-height applied with a pixel values as a unit for one of the following reasons
Altering the height of an element should generally be done with a padding value selected from a ratio based scale that relates to your type scale.
Using line-height to vertically center things is very brittle and only works when there is one line of text. E.g if you have a font-size of 24px and you set line-height to 48px to vertically center the text, if the copy wraps it will have 200% leading which will most likely look very awkward and should thus be avoided
Vertically centering elements should be done by setting vertical-align on an element set to display: table-cell that is contained within an element set to display: table. This is a pattern I will document with coding examples. The benefit being it works in more contexts and can be used to vertically center text elements against media objects across all breakpoints.