Why Does Margin: Auto Not Always Center Positioned Elements?

Margin:auto Not Enough for Centering Positioned Elements

When attempting to horizontally center an absolutely or fixed positioned element, the margin: auto property alone may not suffice.

For in-flow elements (elements without position: absolute or fixed), setting just the width is enough. Margin: auto will automatically balance the left and right margins to center the element.

However, for positioned elements, the situation is different. As per the CSS specification:

• If left, right, and width are all auto, margin-left and margin-right are set to 0, resulting in no centering.
• If left, right, and width are not all auto, margin-left and margin-right are set to equal values, which results in centering.

To center a positioned element:

1. Set left, right, and width

position: absolute;
left: 0;
right: 0;
width: 70px;

2. Set margin-left and margin-right to auto (optional)

position: absolute;
left: 0;
right: 0;
width: 70px;
margin-left: auto;
margin-right: auto;

Setting left and right to 0 essentially defines the left and right boundaries within which the element can move. Margin: auto then balances the space between these boundaries.

Example:

Consider a parent element with a width of 200px and an absolutely positioned child element with a width of 70px:

parent {
  width: 200px;
  position: relative;
}
child {
  position: absolute;
  left: 0;
  right: 0;
  width: 70px;
  margin: auto; /* Optional */
}

When margin: auto is used, the child element is centered horizontally with respect to the parent. This is because the calculated margins (margin-left and margin-right) are equal, which effectively positions the child in the middle of the available space.

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