CSS Box Model
CSS BOX model
content-box - width and height of the element only include content area.
padding-box - Width and height of the element include content and padding.
border-box - Width and height of the element include content, padding, and border.
initial - Sets the box model to its default state.
inherit - Inherits the box model of the parent element.
What is the Box Model?
The browser creates a rectangle for each element in the HTML document. The Box Model describes how the padding, border, and margin are added to the content to create this rectangle.
Diagram from CSS2.2 Working Draft
The perimeter of each of the four areas is called an edge. Each edge defines a box.
- The innermost rectangle is the content box. The width and height of this depend on the element's rendered content (text, images, and any child elements it may have).
- Next is the padding-box, as defined by the padding property. If there is no padding width defined, the padding edge is equal to the content edge.
- Then we have the border-box, as defined by the border property. If there is no border width defined, the border edge is equal to the padding edge.
- The outermost rectangle is the margin box, as defined by the margin property. If there is no margin width defined, the margin edge is equal to the border edge.
This CSS styles all div elements to have a top, right, bottom and left border of 5px in width; a top, right, bottom and left margin of 50px; and a top, right, bottom, and left padding of 20px. Ignoring content, our generated box will look like this:
Screenshot of Google Chrome's Element Styles panel
- As there is no content, the content region (the blue box in the middle) has no height or width (0px by 0px).
- The padding-box by default is the same size as the content box, plus the 20px width on all four edges we're defining above with the padding property (40px by 40px).
- The border-box is the same size as the padding box, plus the 5px width we're defining above with the border property (50px by 50px).
- Finally, the margin box is the same size as the border-box, plus the 50px width we're defining above with the margin property (giving our element a total size of 150px by 150px).
Now let's give our element a sibling with the same style. The browser looks at the Box Model of both elements to work out where in relation to the previous element's content the new element should be positioned:
The content of each element is separated by a 150px gap, but the two elements' boxes touch each other.
The default box model (content-box) can be counter-intuitive since the width/height for an element will not represent its actual width or height on screen as soon as you start adding padding and border styles to the element.
The following example demonstrates this potential issue with content-box:
Since the padding will be added to the width of the text area, the resulting element is a text area that is wider than 100%.
Fortunately, CSS allows us to change the box model with the box-sizing property for an element. There are three different values for the property available:
- content-box: The common box model - width and height only includes the content, not the padding or border
- padding-box: Width and height includes the content and the padding, but not the border
- border-box: Width and height includes the content, the padding as well as the border
To solve the textarea problem above, you could just change the box-sizing property to padding-box or border-box.
border-box is most commonly used.
To apply a specific box model to every element on the page, use the following snippet:
In this coding box-sizing:border-box; is not directly applied to *, so you can easily overwrite this property on individual elements.
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