Why Does a Top Margin on a Child Element Push Down Its Parent Div?

Margin-Top Pushes Parent Div Down: Understanding and Fixing the Issue

Many developers encounter a peculiar issue when applying a top margin to the first visible element on a page: it causes the parent div to be pushed down. To delve into the cause of this behavior and provide a solution, let's analyze the code snippet provided:

div#header {
  width: 100%;
  background-color: #eee;
  position: relative;
}

div#header h1 {
  text-align: center;
  width: 375px;
  height: 50px;
  margin: 50px auto;
  font-size: 220%;
  background: url('../../images/name_logo.png') no-repeat;
}

This code defines a header div with a nested h1 element containing a margin of 50px. Typically, we expect this margin to increase the space between the h1 and the top edge of the header div. However, it instead causes the entire header div to be pushed down by 50px.

To understand why this происходит, we need to consider the "block formatting context". When applied to the first visible element on a page, a top margin resets the block formatting context, causing the parent div to be pulled down.

A solution to this issue is to apply overflow: auto to the parent div. This allows the parent div to automatically adjust its height to accommodate its children elements, including the h1 with the top margin:

div#header {
  width: 100%;
  background-color: #eee;
  position: relative;
  overflow: auto;
}

By adding overflow: auto, we allow the header div to resize vertically, preventing it from being pushed down when the top margin is added to the h1 element.

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