Why Doesn't Percentage Height Work as Expected in CSS?

Why Doesn't Percentage Height Work in CSS?

Understanding the inherent behavior of block elements in CSS is crucial when working with percentage-based dimensions. While percentage widths function as expected, percentage heights exhibit a paradoxical behavior that can leave developers puzzled.

Default Height Determination

By default, the height of a block element is determined by its content. In other words, the element will expand vertically to accommodate the height required by its contents. This is unlike the width property, which inherently expands the element horizontally to fill the available space within its parent.

The Percentage Paradox

When a percentage is used to define an element's height, it signifies a percentage of the parent element's height. However, since the parent element's height is often dependent on the height of its child elements, a feedback loop is created. Setting the child element's height to a percentage does not provide a concrete value to work with, as it relies on the parent's height, which in turn relies on the child's height. This cycle prevents the establishment of a well-defined dimension.

Content-Driven Heights

In contrast to widths, which are independent of content, heights are directly influenced by the content an element contains. This dependency makes it essential to provide a specific height value for the parent element, thus breaking the feedback loop and providing a concrete reference point for the child element's percentage height.

Example to Illustrate

Consider the following code:

<div>
<pre class="brush:php;toolbar:false">#inner {
  height: 50%;
}

In this example, the height of #inner will depend on the height of #outer. However, #outer's height is also dependent on the height of #inner. Without specifying a height for #outer, the feedback loop will prevent #inner from having a well-defined height.