Why Do Absolutely Positioned Elements Sometimes Appear Misaligned?

Understanding Default Positions for Absolutely Positioned Elements

When using the position: absolute property, it's common to encounter issues with misaligned elements. This occurs when the default values for these properties are not as expected.

In HTML, absolutely positioned elements are placed without affecting the surrounding content. However, by default, the values for top, left, bottom, and right are not set to 0.

The Default Values

Instead, the default value for all of these properties is auto. This means that the element's position is calculated based on its containing block, which is the nearest ancestor element with a non-static position.

When all three properties (left, width, and right) are set to auto, the element's position is determined relative to the static position, i.e., its original position if it were not absolutely positioned.

According to the CSS specification (section 10.3.7):

If all three of 'left', 'width', and 'right' are 'auto': First set any 'auto' values for 'margin-left' and 'margin-right' to 0. Then, if the 'direction' property of the element establishing the static-position containing block is 'ltr' set 'left' to the static position and apply rule number three below; otherwise, set 'right' to the static position and apply rule number one below.

Similarly, for vertical dimensions, the properties (top, height, and bottom) are calculated as follows (section 10.6.4):

If all three of 'top', 'height', and 'bottom' are auto, set 'top' to the static position and apply rule number three below.

In simpler terms, the default position is the element's original position in the layout, even though it is absolutely positioned.

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