Why Does "top: 50%" Not Center My Element Vertically When "left: 50%" Works Correctly?

CSS top Percent Not Behaving as Expected: Delving into the Reasons

In pursuit of a responsive layout, you encounter an anomaly where "top:50%" isn't working as anticipated, while "left:50%" functions without issue. Why does this occur?

To comprehend the behavior, consider the following parent and child element structure:

<div>

Key to resolving the problem lies in understanding how the "top" property operates in CSS. When you specify a percentage, such as "50%", it's calculated relative to the height of the parent container. However, in this scenario, the parent container has no explicit height defined, so the "top" property is effectively computed against an unknown value.

To address this, you need to establish a fixed height for the parent container. This will provide the necessary reference point for the child element's top position.

Example 1: Defining a Height

<div>

By providing a height of 200px to the parent div, the child div's top position is now correctly calculated as 50% of 200px, resulting in the expected vertical centering.

Example 2: Stretching the Container

An alternative approach involves stretching the parent container to occupy the entire available space:

<div>

By defining the container's top, bottom, left, and right properties, it expands to fill the available space. The child div's top position is then calculated relative to the stretched container, achieving the same centering effect.

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