css difference between "margin: 20px auto" and "margin: 20px" code example

Example: margin 0 auto

Specifying 0 tells the browser to set the top and bottom margin to 0.

Specifying auto tells the browser to automatically determine the left
and right margins equally.

It's the same as:

margin-top:0;
margin-bottom:0;
margin-left:auto;
margin-right:auto;