Line-height without units

Question

I saw people using line height without specifying a unit, like this: line-height: 1.5;

What does the number represents? I\'m guessing it\'s a ratio so i

Answer 1

See the respective spec @W3C:

The used value of the property is this number multiplied by the element's font size. Negative values are illegal. The computed value is the same as the specified value.

So as you guessed it is a ratio and relates to the current font-size.