Why Does My `margin-top` Percentage Overflow the Container?

Calculating Margin-Top Percentage: Understanding Width-Based Approach

In CSS, vertical margins, including margin-top, are calculated based on the width of the containing block. This approach ensures horizontal and vertical consistency in margin measurements and avoids circular dependencies while laying out content.

Horizontal and Vertical Consistency:

The shorthand margin property allows you to set margins for all four sides. If vertical margins were based on height instead of width, margins on different sides would often have different sizes, breaking the consistent behavior.

Avoiding Circular Dependency:

CSS lays out content in vertical blocks. The width of a block is typically determined by its parent's width. However, the height of a block depends on its content and can change dynamically. If vertical margins depended on height, there would be a circular dependency between the parent's height and the child's margin.

Example:

Consider the following code where the child element has a margin-top: 50%:

<div class="container">
  <p>Some Cool Content</p> 
 
</div>

.container {  
  background: lightblue; 
  padding: 10px; 
  height: 200px;
}

p { 
  display: block; 
  border:1px solid red;
  margin-top:50%;
}

Contrary to the expectation that the child element would be 50% below the top of the container (100px), it overflows the container's height. This is because the percentage margin is based on the width of the container, not its height. In this example, the container's width is 500px, resulting in a margin-top of 250px.

In conclusion, vertical margin calculations are based on the containing block's width to ensure consistency, avoid circular dependencies, and enable efficient layout construction.

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