• The parent of an element has a defined width
• auto says make up the parent's width or total width required for this element by using this value to make up the difference
• margin and the element's width can be specified as auto
• padding and border are set to specified values
• If the width is over-constrained, all set to specific values that do not add up to parent's width, margin-right is forced to auto
• If margin-left and margin-right set to auto they have equal values

p (margin-left:25%;margin-right:25%; width:50%}

A way to centre a paragraph

p (margin-left:auto;margin-right:auto; width:50%}

Correct way to centre a paragraph