Font size with rem

CSS3 introduces a few new units, including the [rem](<https://www.w3.org/TR/css-values/#font-relative-lengths>) unit, which stands for “root em”. Let’s look at how rem works.

First, let’s look at the differences between em and rem.

em: Relative to the font size of the parent. This causes the compounding issue
rem: Relative to the font size of the root or <html> element. This means it’s possible to declare a single font size for the html element and define all rem units to be a percentage of that.

The main issue with using rem for font sizing is that the values are somewhat difficult to use. Here is an example of some common font sizes expressed in rem units, assuming that the base size is 16px :

10px = 0.625rem
12px = 0.75rem
14px = 0.875rem
16px = 1rem (base)
18px = 1.125rem
20px = 1.25rem
24px = 1.5rem
30px = 1.875rem
32px = 2rem

CODE:

html { 
  font-size: 16px; 
}

h1 { 
  font-size: 2rem;          /* 32px */
}

p { 
  font-size: 1rem;          /* 16px */ 
}

li { 
  font-size: 1.5em;         /* 24px */
}