CSS margin collapse: Why does my element margin-top affect the parent element?

Detailed explanation of CSS margin collapse: Cleverly solve margin problems

Many CSS novices often encounter unexpected layout effects when learning margin attributes. This article will use a specific case to deeply analyze the behavior of margin attributes in specific situations, helping you understand and solve the headache-inducing "margin collapse" problem.

Question: Suppose you set the margin-top attribute for an element named "one", but find that not only the "one" element moves downward, but the upper boundary of its parent element "canvas" also moves downward. This does not match the expected results.

(HTML and CSS codes are omitted here, but it is understandable based on the description)

The root cause is margin collapse . Margin collapse refers to when an element has no content (such as text, pictures, etc.) and its vertical margin ( margin-top or margin-bottom ) of its child element is not an auto value, the vertical margin of the parent element will be "merged" with the vertical margin of the child element, resulting in the final displayed margin size not the expected value.

In this example, the parent element of the "one" element "canvas" is not set to the height, and the "one" element contains only one image and no other content. When margin-top: 20px; is set, due to margin collapse, margin-top of "one" is merged with the upper margin of "canvas", causing the overall "canvas" to move downward by 20px.

Solution: There are several ways to avoid margin collapse:

• Set the parent element height: Set a clear height for "canvas", such as height: 100px; to prevent margin collapse.
• Add parent element content: Add a small amount of text or an empty element in "canvas" (e.g.<div></div> ) to prevent the parent element from collapsing highly.
• Using overflow: hidden; : Adding overflow: hidden; attribute to the parent element can also effectively solve this problem.
• Leverage BFC (block-level formatting context): Setting the parent or child element to BFC can prevent margin collapse. For example, use properties such as display: flex; or display: inline-block;

Mastering the mechanism of margin collapse and applying appropriate solutions will allow you to better control the layout of page elements and avoid similar problems.

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