Analysis of usage and common problems of margin in CSS

We are familiar with margin. This article mainly introduces the usage of margin in CSS and the analysis of common problems. The introduction in the article is very detailed and has certain reference and learning value for everyone. Friends who need it can follow Let’s take a look together.

1. Introduction

marginWe are generally used to calling it margins. We can set the margins in four directions respectively. No more details here. Describe the assignment syntax in detail.

Actually, the margins we usually set are physical-level settings, and margins also include start, end, before, after, etc. These are mainly logical-level settings. If you are interested, Google it yourself. .

When setting margin, we must know:

• For block-level elements, how effective is margin in four directions;

• For inline elements, margin is only effective in the horizontal direction.
  

2. Box model

Speaking of margin, I have to talk about the box model:

1. Content => padding => border => margin

The reason why the box model needs to be understood from the inside and outside is mainly because the standards of the box model are different. It determines what the width we set in CSS is. At this time, everyone will think of those calculation formulas. In fact, with the arrival of CSS3, we can set the standard of the box model through box-sizing:

2. border-box: width is calculated from border;

3. content-box: width is calculated from content;

4. padding-box: has been calculated from removed from the standard.

Here’s another picture, are you already aware of it?

##3. Margin overlap problem

This kind of problem mainly occurs on block elements and not floating elements (it is not described clearly here, and will be discussed in detail later). Let’s take a look at what happened.

1. Occurs in adjacent sibling elements

    .a {
        margin: 50px 0;
    }
    .b {
        margin: 100px 0;
    }

Adjacent sibling Margin overlap of elements

In this case, margin overlap occurs. The distance between adjacent sibling nodes is the maximum margin value. The best way to avoid this situation is to set only margin-top or margin-bottom.

2. Occurs at the parent-child node

    p(class="b")
    p(class="a")
        p(class="c") C

    .a {
        margin: 20px 0;
    }
    .b {
        margin: 100px 0;
    }

Margin overlap occurs between parent and child nodes

According to our understanding here, the distance between a and b should be 20 pixels, and the distance between c and a is 100 pixels. But this is not the case. Here we can solve the overlapping problem through the following methods:

• The parent element sets the border;

• The parent element sets the padding ;

• The parent element sets overflow to hidden or scroll, others are not applicable;

• The parent element sets position to fixed or absolute, others not applicable.

4. Magical negative margin value

We set the margin in four directions for a block element What happens:

• Setting negative values ​​for top and left will move the element up or to the left by the corresponding pixel distance;

• setting bottom and right Negative values ​​will move adjacent elements up or to the left.

There is another point here. When I checked the information, I found that many people said that negative margin can change the width of the element. I would like to correct it here. This is not a characteristic of negative margin. , but the characteristics of margin. For example, the following CSS can completely set the width of block-level elements.

    .item {
        margin: 0 200px;
        height: 200px;
    }

Margin assignment is widely used in layout. For example, if we know the width and height of an element, you can center it through negative margin values.

    .item {
        position: absolute;
        background: red;
        width: 200px;
        height: 200px;
        top: 50%;
        left: 50%;
        margin-top: -100px;
        margin-left: -100px;
    }

5. Margin and float

For these two troublesome attributes, Together, let me just say, it was a real blast. I said above that "this problem mainly occurs on block elements and not floating elements". Here are two more points:

• Adjacent sibling elements, if both of them are floating element, no margin overlap will occur;

• For parent-child elements, if one of them is a floating element, no margin overlap will occur;

  
  

Mainly because the floating elements are not in the normal document flow, so it is better to use the method of clearing the floats. (The following is the implementation of bootstrap)

    .clearfix::before, .clearfix::after {
        content: " ";
        display: table;
    }

    .clearfix::after {
        clear: both;
    }

学完本文相信大家对css 中margin属性有更深的认识，大家赶紧收藏起来吧。

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