In CSS, how does the auto attribute in 'margin: 0 auto' work?-HTML Tutorial-php.cn

In CSS, how does the auto attribute in 'margin: 0 auto' work?

王林

Release： 2023-08-28 10:57:02

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在CSS中，"margin: 0 auto"中的auto属性是如何工作的？

The "margin: 0 auto" property is a commonly used CSS property that allows developers to center elements horizontally within a container. The "auto" value of the margin attribute enables the centering effect to be achieved.

In this article, we will explore how the "auto" value works in the margin property and how to use it to achieve horizontal centering. We'll also discuss some possible errors and best practices when using the "auto" value in margin properties.

Margins in CSS

Before entering this topic, we should try to learn some basic knowledge in order to understand this problem. First, we will learn what margin means in CSS, and then we will move on to understanding the auto attribute. Only after learning all this can we arrive at the answer to our original question.

We know that the purpose of CSS is to style web pages so that they are easy to use and visually pleasing, making the overall user experience smoother and better for users. This style includes many things such as color, font, font size, etc. One of the styling methods is to use appropriate spacing between elements.
Whenever we are talking about the space outside the defined bounds of an element, we are actually talking about its margins. Margins allow us to create an invisible border that isolates an element from other elements. It's a bit like padding, but padding is actually the space between an element's children and surrounding elements.
CSS allows us to have a very high level of control and customization over the margins of an element, we can use margin normally to create a margin that is equal on all four sides, but we can also create a margin that is equal on all four sides by specifying that we are actually referencing margins to individually define margins for specific sides. For example,

margin : 0 // statement 1
margin top : 25px // statement 2

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The margin set by statement 1 will set the margins of the elements to 0, while the margin set by statement 2 will only modify the top margin and set it to 25 pixels.

We can specify margins in different ways -

We can use custom length.
We can specify a percentage value that will change based on the user's screen size.
We can also make margins inheritable, which will set the current element's margins to those of its parent element.

But what if we don’t know what value we should use when defining the margin.

Automatic properties

CSS provides us with an attribute so that the browser can calculate and set the margin of the element. This attribute is the auto attribute. This property makes it easier for developers to use margins because we don't need to know in advance the actual value that will be used, but let the browser calculate it.

Let's first understand how it works. If we specify the margins of an element as auto, it will give equal margins on all sides by first calculating the width and size of the element.

The Chinese translation of

Example

is:

Example

Let's consider a div with dimensions of 500 pixels by 300 pixels. If we don't specify any margins, it will automatically align to the top left corner of the screen. On the other hand, specifying margins as auto will center it within the parent container, in this case the body tag.

<!DOCTYPE html>
<html lang="en">
<head>
   <title>Margin: 0 auto example</title>
</head>
<body style="background-color: coral; margin: 0; padding: 0; font-size: larger;
font-weight: bold;">
   <div style="width: 500px; height: 300px; border: 1px solid black; background-color: aqua;">No set margin</div>
   <div style="width: 500px; height: 300px; border: 1px solid black; background-color: burlywood; margin: auto;">Margin set to 0</div>
</body>
</html>

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Use margins: 0 automatic

Now, what happens if we try to set the margin using two values? Whenever we try to use margins and provide two values, the first value is considered as the value for the top and bottom margins, while the second value is used for the left and right margins.

Our question is to explain, "How will it work if we use the auto attribute as the second value of the margin.

The answer is, "It will vertically align the element to the center of its parent element by automatically calculating the left and right margins."

The Chinese translation of

Example

is:

Example

Consider the same as the above example, but set the margins to 0 auto.

<!DOCTYPE html>
<html lang="en">
<head>
   <title>Margin: 0 auto example</title>
</head>
<body style="background-color: coral; margin: 0; padding: 0; font-size: larger; font-weight: bold;">
   <div style="width: 500px; height: 300px; border: 1px solid black; background-color: burlywood; margin: 0 auto;">Margin set to 0 auto</div>
</body>
</html>

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in conclusion

In this article, we learned about the meaning of margin in CSS, the role of auto property in CSS and how its behavior changes when we use it as the second value of margin. Our initial answer was to vertically align the element with its parent by automatically calculating the left and right margins.

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