What is the box-sizing: border-box property in HTML5 layouts?
box-sizing: border-box is to make the width and height of the element contain content, margins and borders, thereby achieving a more predictable layout; 1. In the default content-box model, the width only refers to the content area, and padding and border will add additional total size; 2. After using border-box, the set width is the total width of the element, and the content area is automatically compressed; 3. This makes responsive design simpler, without manually calculating the impact of padding and border; 4. Developers often use * { box-sizing: border-box; } to unify layout behavior; therefore, box-sizing: border-box has become a standard practice in modern CSS layout.
The box-sizing: border-box property in CSS (not HTML5, since it's a styling feature) changes how the width and height of elements are calculated in web layouts. It's a key tool for making layout design more predictable and easier to manage.
By default, most elements use box-sizing: content-box , which means:
- The
widthandheightyou set apply only to the content area. - Padding and borders are added on top of the declared width.
For example:
.box {
width: 200px;
padding: 20px;
border: 5px solid black;
} With content-box , the total width becomes: 200px (content) 40px (left/right padding) 10px (left/right border) = 250px .
That can make responsive layouts tricky, because you have to manually account for padding and borders when trying to fit elements into containers.
How border-box changes things
When you set:
box-sizing: border-box;
Now:
- The
widthincludes the content, padding, and border. - If you set
width: 200px, the entire element (content padding border) will be exactly 200px wide. - The content area shrinks to make room for padding and borders.
So the same example with border-box :
.box {
box-sizing: border-box;
width: 200px;
padding: 20px;
border: 5px solid black;
}Total width = 200px (as specified). The content area is now only 150px wide (200 - 40px padding - 10px borders).
Why it's useful in layouts
- Easier responsive design : You can use percentages and know that padding won't push elements out of their containers.
- Simpler math : No need to subtract padding and borders when calculating widths.
- Consistent sizing : Columns in a flex or grid layout behave more predictably.
Many developers apply it globally:
* {
box-sizing: border-box;
}This way, every element uses the more innovative sizing model by default.
In short, box-sizing: border-box makes element dimensions including padding and borders, leading to more predictable and manageable layouts. It's become a standard practice in modern CSS.
The above is the detailed content of What is the box-sizing: border-box property in HTML5 layouts?. For more information, please follow other related articles on the PHP Chinese website!
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