Why Doesn't 'top: 50%' Work as Expected in CSS?

CSS Percentage Offset: Understanding Why "top: 50%" Isn't Working

In the realm of responsive CSS layouts, setting a percentage value for the "top" property can sometimes prove challenging. While "left: 50%" operates as expected, using "top: 50%" yields unexpected results.

原因

The key lies in understanding how percentages for the "top" property are calculated. Unlike "left", which references the container's width, "top" refers to the container's height. Therefore, if the container's height is not defined, the "top: 50%" value effectively becomes 50% of 0px, resulting in no vertical displacement.

Solution

To resolve this issue, there are two primary approaches:

1. Define a Dimension for the Parent Container:
   Specify explicit height and width values for the container to provide a reference for percentage-based offsets. For instance:

   <div>

2. Stretch the Parent Container:
   Alternatively, define the container's height and width using absolute values and stretch it to the edges of its containing element:

   <div>

By implementing one of these solutions, you can ensure that "top: 50%" functions as intended, providing vertical centering within a responsive layout.

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