Font-Relative Units
Let's take a look at CSS units relative to font, such as em, rem, ex, and ch.
We'll cover the following...
The  em unit
.element {
  width: 30em; 
}
The  em unit is the value assigned to the element’s font-size,  and its exact value changes between elements. The measurement itself is the width of the letter “m.”
The length changes only if we change the font-size. It doesn’t change if the font-family is ...
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