Introduction to methods of margin overlap and prevention in CSS (code example)

This article brings you an introduction to methods of margin overlap and prevention in CSS (code examples). It has certain reference value. Friends in need can refer to it. I hope it will help You helped.

Margin Overlapping Solution (BFC)

First of all, we must clarify what BFC means. Its full English spelling is Block Formatting Context, which is literally translated as "block-level formatting context"

The vertically adjacent boundaries of two or more block-level boxes will overlap. The resulting border width is the largest of adjacent border widths. If a negative boundary occurs, the negative boundary with the largest absolute value is subtracted from the largest positive boundary. If there are no positive boundaries, the negative boundary with the largest absolute value is subtracted from zero. Note: Adjacent boxes may not be generated by elements with parent-child or sibling relationships.

But there are exceptions to the overlap of borders:

1. The horizontal margins will never overlap.
2. In the specification document, the adjacent vertical margins of two or more block-level box models will overlap. The final margin value is calculated as follows:
a. If all are positive values, take the largest one;
b. If not all are positive values, take the absolute value, and then subtract the maximum value from the positive value;
c. If there is no positive value, take the absolute value, and then subtract the maximum value from 0.
Note: Adjacent box models may be generated dynamically from DOM elements and have no adjacent or inheritance relationship.
3. If one of the adjacent box models is floating (float), the vertical margins will not overlap, and the same is true between the floating box model and its child elements.
4. The margin between the element with the overflow attribute set and its child elements will not overlap (except when the overflow value is visible).
5. For box models with absolute positioning (position:absolute) set, vertical margins will not overlap, and the same will be true with their child elements.
6. For elements with display:inline-block set, vertical margins will not overlap, even with their child elements.
7. If the upper and lower margins of a box model are adjacent, its margins may collapse through it. In this case, the position of the element depends on whether the margins of its neighboring elements overlap.
a. If the margin of an element overlaps the margin-top of its parent element, the border definition of the box model border-top is the same as that of its parent element.
b. In addition, the parent element of any element does not participate in the overlap of margin, or only the margin-bottom of the parent element participates in the calculation. If the element's border-top is non-zero, then the element's border-top position is the same as before.
The margin-top of an element to which clearing is applied will never overlap the margin-bottom of its block-level parent element.
Note that the position of elements that have been covered by overlap has no effect on the position of other overlapped elements; the border-top border position is only necessary when positioning the child elements of these elements.
8. The vertical margin of the root element will not be overlapped.

Example of margin overlap
Margin overlap refers to two vertically adjacent block-level elements. When the upper and lower margins meet, the margins will overlap and overlap. The subsequent margin is equal to the larger of them.
Illustration:

Another overlapping phenomenon is that when an element is included in another element, there will also be overlap between the child element and the parent element. The margin after overlap is equal to The largest one:

#Similarly, if an empty element has no content, its top and bottom margins will also overlap.

The meaning of margin overlap

The overlap of margins only occurs between the upper and lower margins of ordinary flow documents. This seems a bit strange rule. , in fact, it has practical significance. Imagine that when we arrange a series of regular block-level elements (such as paragraph P) up and down, then due to the overlapping margins between the block elements, there will be no double distance between paragraphs.

Case: As shown below, the parent element does not set margin-top, but the child element sets margin-top: 20px; it can be seen that the parent element also has margins.

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        *{
            margin:0;
            padding:0;
        }
        .demo{
            height:100px;
            background: #eee;
        }
        .parent{
            height:200px;
            background: #88f;
        }
        .child{
            height:100px;
            margin-top:20px;
            background: #0ff;
            width:200px;
        }
    </style>
</head>
<body>
    <section>
        <h2 id="此部分是能更容易看出让下面的块的margin-top">此部分是能更容易看出让下面的块的margin-top。</h2>
    </section>
    <section>
        <article>
            <h2 id="子元素">子元素</h2>
            margin-top:20px;
        </article>
        <h2 id="父元素">父元素</h2>
            没有设置margin-top
    </section>
</body>
</html>

Solution to this problem: set padding-top on the parent element :20px or set overflow:hidden, and cancel margin-top:20px for child elements;

to prevent external margins Overlapping solution:

Although the overlap of margins has a certain meaning, sometimes we don’t want overlap between elements in the design, so we can have the following suggestions for reference:

1. Outer element padding instead

2. Inner element transparent border: 1px solid transparent;

3. Inner element absolute positioning postion:absolute:

4. Outer element overflow:hidden;

5. Inner element, add float: left; or display: inline-block;

6. Inner element padding: 1px;

The above suggestions can be adopted according to the actual situation.

【Related recommendations: CSS video tutorial】

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