Why em instead of px?

Question

I heard you should define sizes and distances in your stylesheet with em instead of in pixels. So the question is why should I use em instead of px when defining styles in c

Answer 1

example:

Code: body{font-size:10px;} //keep at 10 all sizes below correct, change this value and the rest change to be e.g. 1.4 of this value

1{font-size:1.2em;} //12px

2{font-size:1.4em;} //14px

3{font-size:1.6em;} //16px

4{font-size:1.8em;} //18px

5{font-size:2em;} //20px

…

body

1

2

3

4

5

by changing the value in body the rest change automatically to be a kind of times the base value…

10×2=20 10×1.6=16 etc

you could have the base value as 8px… so 8×2=16 8×1.6=12.8 //may be rounded by browser