Web page elements occupy space through the CSS box model, each element consisting of content, inner margins, borders, and outer margins. 1. The content area is the core area, and the size is set through width and height; 2. The padding surrounds the content and increases the overall size of the element; 3. The border defines the box outline, which will also affect the size; 4. The margin controls the distance from other elements, which may merge and affect the layout. Use box-sizing: border-box; let width and height include padding and border, which facilitates control of the total size. Understanding these four parts and their relationships is the key to mastering web page layout.

How do web elements occupy space in page layout? The answer lies in the box model of CSS. Each HTML element is essentially a box, and this "box" consists of four parts: content, padding, border and margin. Understanding their respective roles is the basis for mastering web page layout.

Content: The core of the box

The content area is where you put text, pictures or other elements, which determines the most basic size of the box. You can set the size of the content area through width and height .

For example:

 .box {
  width: 200px;
  height: 100px;
}

The above code sets the width and height of the content area, but the entire element actually takes up more than this space - because there are padding, border and margin.

By default, the browser uses the standard box model (i.e. content-box), which means that the width and height you set only correspond to the content area and do not include other parts.

If you want width and height to include padding and border, you can add this sentence:

 box-sizing: border-box;

Padding: The gap between content and border

Padding is a blank area around content. Its function is to prevent the content from sticking to the border and improve visual comfort.

Common usage:

• padding: 10px; add 10px in all four directions
• padding-left: 20px; only add inner margin to the left

Note: padding affects the overall width and height of the entire box. For example, if the content is 200px wide, the padding is 10px each, then this part becomes 220px.

Border: Box boundary line

Border is the "outline" of a box, which can be color, style, and width. While it doesn't take up much space by itself, it does add to the overall size of the box.

Basic writing method:

 border: 2px solid #333;

You can set the borders on one side separately, for example:

• border-top: 1px dashed red;
• border-right-width: 5px;

Just like padding, borders also increase the overall width and height of the box. If you use box-sizing: border-box; you don't need to worry about this problem.

Margin: Controls the distance from other elements

Margin is the distance between the box and other elements. It is transparent and will not have any background color or border, but is just used as a "separation belt".

Common Tips:

• margin: 0 auto; centered block-level element
• margin-bottom: 20px; controls the vertical spacing between elements

It is worth noting that margin collapses in two adjacent vertical directions, which may bring unexpected effects when laying out, so special attention should be paid.

Basically that's it. The CSS box model looks simple, but is very critical in the actual layout. Many beginners encounter questions such as "Why is my div bigger than expected?" and "Why doesn't margin work?", which can often be found from the perspective of box models. Just remember the relationship between the four parts and make good use of box-sizing , you can control the layout of the web page more freely.

The above is the detailed content of Understanding the css box model: content, padding, border, margin. For more information, please follow other related articles on the PHP Chinese website!