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The CSS Box Model
For explaining how to use CSS to arrange objects (elements) we have understand the CSS Box Model. HTML elements can be considered as boxes. In CSS, the term "box model" is used when talking about design and layout. The CSS box model is essentially a box that wraps around HTML elements, and it consists of: margins, borders, padding, and the actual content. The box model allows us to place a border around elements and space elements in relation to other elements. The image below illustrates the box model:
Explanation of the different parts:
- Margin - Clears an area around the border. The margin does not have a background color, it is completely transparent
- Border - A border that goes around the padding and content. The border is affected by the background color of the box
- Padding - Clears an area around the content. The padding is affected by the background color of the box
- Content - The content of the box, where text and images appear In order to set the width and height of an element correctly in all browsers, you need to know how the box model works.
Width and Height of an Element Important: When you specify the width and height properties of an element with CSS, you are just setting the width and height of the content area. To know the full size of the element, you must also add the padding, border and margin. The total width of the element in the example below is 300px:
width:250px; padding:10px; border:5px solid gray; margin:10px;
Let's do the math: 250px (width)
- 20px (left and right padding)
- 10px (left and right border)
- 20px (left and right margin) = 300px Imagine that you only had 250px of space. Let's make an element with a total width of 250px:
Example
width:220px; padding:10px; border:5px solid gray; margin:0px;
The total width of an element should always be calculated like this: Total element width = width + left padding + right padding + left border + right border + left margin + right margin The total height of an element should always be calculated like this: Total element height = height + top padding + bottom padding + top border + bottom border + top margin + bottom margin
Browsers Compatibility Issue If you tested the previous example in Internet Explorer, you saw that the total width was not exactly 250px. IE includes padding and border in the width, when the width property is set, unless a DOCTYPE is declared. To fix this problem, just add a DOCTYPE to the code:
Example
Border Color The border-color property is used to set the color of the border. The color can be set by:
- name - specify a color name, like "red"
- RGB - specify a RGB value, like "rgb(255,0,0)"
- Hex - specify a hex value, like "#ff0000" You can also set the border color to "transparent". Note: The "border-color" property does not work if it is used alone. Use the "border-style" property to set the borders first.
Example
p.one { border-style:solid; border-color:red; } p.two { border-style:solid; border-color:#98bf21; }
Border - Individual sides In CSS it is possible to specify different borders for different sides:
Example
p { border-top-style:dotted; border-right-style:solid; border-bottom-style:dotted; border-left-style:solid; }
The example above can also be set with a single property:
Example
border-style:dotted solid;
The border-style property can have from one to four values.
- border-style:dotted solid double dashed; o top border is dotted o right border is solid o bottom border is double o left border is dashed
- border-style:dotted solid double; o top border is dotted o right and left borders are solid o bottom border is double
- border-style:dotted solid;
o top and bottom borders are dotted o right and left borders are solid
- border-style:dotted; o all four borders are dotted The border-style property is used in the example above. However, it also works with border- width and border-color.
CSS Outlines
An outline is a line that is drawn around elements, outside the border edge, to make the element "stand out". The outline properties specifies the style, color, and width of an outline.
Example
border:1px solid red; outline-style:dotted; outline-width:thick; outline-color:green;
CSS Margin
The CSS margin properties define the space around elements.
Margin
The margin clears an area around an element (outside the border). The margin does not have a background color, and is completely transparent. The top, right, bottom, and left margin can be changed independently using separate properties. A shorthand margin property can also be used, to change all margins at once.
Possible Values
Value Description Auto The browser sets the margin. The result of this is dependant of the browser
length Defines a fixed margin (in pixels, pt, em, etc.) % Defines a margin in % of the containing element
Note: It is possible to use negative values, to overlap content.
CSS Padding
The CSS padding properties define the space between the element border and the element content.
Padding
The padding clears an area around the content (inside the border) of an element. The padding is affected by the background color of the element. The top, right, bottom, and left padding can be changed independently using separate properties. A shorthand padding property can also be used, to change all paddings at once.
Possible Values
Value Description
length Defines a fixed padding (in pixels, pt, em, etc.)
% Defines a padding in % of the containing element
Padding - Individual sides
In CSS, it is possible to specify different padding for different sides:
Example
padding-top:25px; padding-bottom:25px; padding-right:50px; padding-left:50px;
Padding - Shorthand property
To shorten the code, it is possible to specify all the padding properties in one property. This is called a shorthand property. The shorthand property for all the padding properties is "padding":
Example
padding:25px 50px;
The padding property can have from one to four values.
- padding:25px 50px 75px 100px; o top padding is 25px o right padding is 50px o bottom padding is 75px o left padding is 100px
- padding:25px 50px 75px; o top padding is 25px o right and left paddings are 50px o bottom padding is 75px
- padding:25px 50px; o top and bottom paddings are 25px o right and left paddings are 50px
- padding:25px; o all four paddings are 25px
Borders
No border.
A dotted border.
A dashed border.
A solid border.
A double border.
A groove border.
A ridge border.
An inset border.
An outset border.
A hidden border.
Outlines
Note: Internet Explorer 8 (and higher) supports the outline property if a !DOCTYPE is specified.
Note: Internet Explorer 8 (and higher) supports the outline property if a !DOCTYPE is specified.
Margins
This is a paragraph with no specified margins.
This is a paragraph with specified margins.
Paddings
This is a paragraph with no specified padding.
This is a paragraph with specified paddings.
Source: w3schools.com
