Margins

Negative values are allowed.

div {
  width: 300px;
  margin: auto;  //horizontally center
  border: 1px solid red;
}

Margin Collapse

Top and bottom margins of elements are sometimes collapsed into a single margin that is equal to the largest of the two margins.

This does not happen on left and right margins! Only top and bottom margins!

Paddings

Negative values are not allowed.

Use the box-sizing property to keep the width at 300px, no matter the amount of padding:

div {
  width: 300px;
  padding: 25px;
  box-sizing: border-box;
}