Check out example codes for "em". It will help you in understanding the concepts better.

Code Example 1

/*1em = 16*/
/*Best solution*/
body
/*THis will set the body font to default size which is commonly 16px*/
{
  font-size: 100%
}
/*Now you adjust element font-size according to the body*/
p
{
  font-size: .8em
}

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