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The CSS box model describes the rectangular boxes that are
generated for elements in the document
tree and laid out according to the visual formatting
model. The page box is a special kind of
box that is described in detail in the section on paged media.
Each box has a
content area (e.g.,
text, an image, etc.) and optional surrounding
padding,
border, and
margin areas; the size
of each area is specified by properties defined below. The following
diagram shows how these areas relate and the terminology used to refer
to pieces of margin, border, and padding:
The margin, border, and padding can be broken down into left,
right, top, and bottom segments (e.g., in the diagram, "LM" for left
margin, "RP" for right padding, "TB" for top border, etc.).
The perimeter of each of the four areas (content, padding, border,
and margin) is called an "edge", so each box has four edges:
- content edge
or inner edge
- The content edge surrounds the element's rendered content.
- padding edge
- The padding edge surrounds the box padding. If the padding
has 0 width, the padding edge is the same as the content edge.
The padding edge of a box defines the edges of the
containing block established by the box.
- border edge
- The border edge surrounds the box's border. If the border
has 0 width, the border edge is the same as the padding edge.
- margin edge
or outer
edge
- The margin edge surrounds the box margin. If the margin
has 0 width, the margin edge is the same as the border edge.
Each edge may be broken down into a left, right, top, and bottom
edge.
The dimensions of the content area of a box -- the content width and content height --
depend on several factors: whether the element generating
the box has the 'width'
or 'height' property
set, whether the box contains text or other boxes, whether the
box is a table, etc. Box widths and heights are discussed
in the chapter on visual formatting
model details.
The box width is given by the
sum of the left and right margins, border, and padding, and the
content width. The height is given by the sum
of the top and bottom margins, border, and padding, and the content
height.
The background style of the various areas of a box are determined
as follows:
- Content area: The 'background' property of
the generating element.
- Padding area: The 'background' property of
the generating element.
- Border area: The
border properties of the generating element.
- Margin area: Margins are always transparent.
This example illustrates how margins, padding, and borders
interact. The example HTML document:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<TITLE>Examples of margins, padding, and borders</TITLE>
<STYLE type="text/css">
UL {
background: green;
margin: 12px 12px 12px 12px;
padding: 3px 3px 3px 3px;
/* No borders set */
}
LI {
color: black; /* text color is black */
background: gray; /* Content, padding will be gray */
margin: 12px 12px 12px 12px;
padding: 12px 0px 12px 12px; /* Note 0px padding right */
list-style: none /* no glyphs before a list item */
/* No borders set */
}
LI.withborder {
border-style: dashed;
border-width: medium; /* sets border width on all sides */
border-color: black;
}
</STYLE>
</HEAD>
<BODY>
<UL>
<LI>First element of list
<LI class="withborder">Second element of list is longer
to illustrate wrapping.
</UL>
</BODY>
</HTML>
results in a document tree with
(among other relationships) a UL element that has two LI
children.
The first of the following diagrams illustrates what this example
would produce. The second illustrates the relationship between the
margins, padding, and borders of the UL elements and those of its
children LI elements.
Note that:
- The content width for each LI box is
calculated top-down; the containing
block for each LI box is established by the UL element.
- The height of each LI box is given by its content height, plus top and bottom
padding, borders, and margins. Note that vertical margins between the
LI boxes collapse.
- The right padding of the LI boxes has been set to zero width
(the 'padding' property). The
effect is apparent in the second illustration.
- The margins of the LI boxes are transparent -- margins are always
transparent -- so the background color (green) of the UL padding and
content areas shines through them.
- The second LI element specifies a dashed border (the
'border-style' property).
Margin properties specify the width of the margin area of a box. The
'margin' shorthand property sets
the margin for all four sides while the other margin properties only
set their respective side.
The properties defined in this section refer to the <margin-width>
value type, which may take one of the following values:
- <length>
- Specifies a fixed width.
- <percentage>
- The percentage is calculated
with respect to the width of the generated box's
containing block.
This is true for 'margin-top' and 'margin-bottom',
except in the page context,
where percentages refer to page box height.
- auto
- See the section on computing widths and
margins for behavior.
Negative values for margin properties are allowed, but there may be
implementation-specific limits.
These properties set the top, right, bottom, and left margin of a
box.
Example(s):
H1 { margin-top: 2em }
-
'margin'
-
| Value: | <margin-width>{1,4} | inherit
| | Initial: | not defined for shorthand properties
| | Applies to: | all elements
| | Inherited: | no
| | Percentages: | refer to width of containing block
| | Media: | visual
|
The 'margin' property is a
shorthand property for setting 'margin-top', 'margin-right', 'margin-bottom', and 'margin-left' at the same place in
the style sheet.
If there is only one value, it applies to all
sides. If there are two values, the top and bottom margins
are set to the first value and the right and left margins are
set to the second. If there are three values, the top is
set to the first value, the left and right are set to the
second, and the bottom is set to the third. If there are
four values, they apply to the top, right, bottom, and left,
respectively.
Example(s):
BODY { margin: 2em } /* all margins set to 2em */
BODY { margin: 1em 2em } /* top & bottom = 1em, right & left = 2em */
BODY { margin: 1em 2em 3em } /* top=1em, right=2em, bottom=3em, left=2em */
The last rule of the example above is equivalent to the example
below:
BODY {
margin-top: 1em;
margin-right: 2em;
margin-bottom: 3em;
margin-left: 2em; /* copied from opposite side (right) */
}
In this specification, the expression collapsing margins means
that adjoining margins (no padding or border areas separate them) of
two or more boxes (which may be next to one another or nested) combine
to form a single margin.
In CSS2, horizontal margins never collapse.
Vertical margins may collapse between certain boxes:
- Two or more adjoining vertical margins of block boxes in the normal flow collapse. The
resulting margin width is the maximum of the adjoining margin widths.
In the case of negative margins, the absolute maximum of the negative
adjoining margins is deducted from the maximum of the positive
adjoining margins. If there are no positive margins, the absolute
maximum of the negative adjoining margins is deducted from zero.
- Vertical margins between a floated box and any other box do not
collapse.
- Margins of absolutely and
relatively positioned boxes do not collapse.
Please
consult the examples of margin, padding, and
borders for an illustration of collapsed margins.
The padding properties specify the width of the padding area of a box. The 'padding' shorthand property sets the
padding for all four sides while the other padding properties only set
their respective side.
The properties defined in this section refer to the <padding-width>
value type, which may take one of the following values:
- <length>
- Specifies a fixed width.
- <percentage>
- The percentage is calculated with
respect to the width of the generated box's containing block, even for
'padding-top' and 'padding-bottom'.
Unlike margin properties, values for padding values cannot be
negative. Like margin properties, percentage values for padding
properties refer to the width of the generated box's containing block.
These properties set the top, right, bottom, and left padding of
a box.
Example(s):
BLOCKQUOTE { padding-top: 0.3em }
-
'padding'
-
| Value: | <padding-width>{1,4} | inherit
| | Initial: | not defined for shorthand properties
| | Applies to: | all elements
| | Inherited: | no
| | Percentages: | refer to width of containing block
| | Media: | visual
|
The 'padding' property is a
shorthand property for setting 'padding-top', 'padding-right', 'padding-bottom', and 'padding-left' at the same place
in the style sheet.
If there is only one value, it applies to all
sides. If there are two values, the top and bottom paddings
are set to the first value and the right and left paddings are
set to the second. If there are three values, the top is
set to the first value, the left and right are set to the
second, and the bottom is set to the third. If there are
four values, they apply to the top, right, bottom, and left,
respectively.
The surface color or image of the padding area is specified via
the 'background' property:
The border properties specify the width, color, and style of the border area of a box. These
properties apply to all elements.
Note.
Notably for HTML,
user agents may render borders for certain elements (e.g.,
buttons, menus, etc.) differently than for
"ordinary" elements.
The border width properties specify the width of the border area. The properties
defined in this section refer to the <border-width>
value type, which may take one of the following values:
- thin
- A thin border.
- medium
- A medium border.
- thick
- A thick border.
- <length>
- The border's thickness has an explicit value. Explicit
border widths cannot be negative.
The interpretation of the first three values depends on the user
agent. The following relationships must hold, however:
'thin' <='medium' <= 'thick'.
Furthermore, these widths must be constant throughout a document.
These properties set the width of the top, right, bottom,
and left border of a box.
This property is a shorthand property for setting
'border-top-width',
'border-right-width',
'border-bottom-width',
and
'border-left-width' at
the same place in the style sheet.
If there is only one value, it applies to all
sides. If there are two values, the top and bottom borders
are set to the first value and the right and left are
set to the second. If there are three values, the top is
set to the first value, the left and right are set to the
second, and the bottom is set to the third. If there are
four values, they apply to the top, right, bottom, and left,
respectively.
Example(s):
In the examples below, the comments indicate the resulting widths
of the top, right, bottom, and left borders:
H1 { border-width: thin } /* thin thin thin thin */
H1 { border-width: thin thick } /* thin thick thin thick */
H1 { border-width: thin thick medium } /* thin thick medium thick */
The border color properties specify the color of a box's border.
The 'border-color'
property sets the color of the four borders. Values have the following
meanings:
- <color>
- Specifies a color value.
- transparent
- The border is transparent (though it may have width).
The 'border-color'
property can have from one to four values, and the values are set on
the different sides as for 'border-width'.
If an element's border color is not specified
with a border property, user agents must use the value
of the element's 'color' property as the
computed value
for the border color.
Example(s):
In this example, the border will be a solid black line.
P {
color: black;
background: white;
border: solid;
}
The border style properties specify the line style of a box's
border (solid, double, dashed, etc.). The properties defined in this
section refer to the <border-style>
value type, which make take one of the following:
- none
- No border. This value forces
the computed value of 'border-width'
to be '0'.
- hidden
- Same as 'none', except in terms of border conflict
resolution for table elements.
- dotted
- The border is a series of dots.
- dashed
- The border is a series of short line segments.
- solid
- The border is a single line segment.
- double
- The border is two solid lines. The sum of
the two lines and the space between them
equals the value of 'border-width'.
- groove
- The border looks as though it were carved
into the canvas.
- ridge
- The opposite of 'grove': the border
looks as though it were coming out of the canvas.
- inset
- The border makes the entire box look as though
it were embedded in the canvas.
- outset
- The opposite of 'inset': the
border makes the entire box look as though
it were coming out of the canvas.
All borders are drawn on top of the box's background. The color of
borders drawn for values of 'groove', 'ridge', 'inset', and 'outset'
depends on the element's 'color'
property.
Conforming HTML user agents may
interpret 'dotted', 'dashed', 'double', 'groove', 'ridge',
'inset', and 'outset' to be 'solid'.
The 'border-style'
property sets the style of the four borders. It can have from one to
four values, and the values are set on the different sides as for
'border-width' above.
Example(s):
#xy34 { border-style: solid dotted }
In the above example, the horizontal borders will be 'solid' and
the vertical borders will be 'dotted'.
Since the initial value of the border styles is 'none', no borders
will be visible unless the border style is set.
This is a shorthand property for setting the width, style, and
color of the top, right, bottom, and left border of a box.
Example(s):
H1 { border-bottom: thick solid red }
The above rule will set the width, style, and color of the border
below the H1 element. Omitted values are set to
their initial values. Since
the following rule does not specify a border color, the border will
have the color specified by the
'color' property:
H1 { border-bottom: thick solid }
The 'border' property is a
shorthand property for setting the same width, color, and style for
all four borders of a box. Unlike the shorthand 'margin' and 'padding' properties, the 'border' property cannot set different
values on the four borders. To do so, one or more of the other border
properties must be used.
Example(s):
For example, the first rule below is
equivalent to the set of four rules shown after it:
P { border: solid red }
P {
border-top: solid red;
border-right: solid red;
border-bottom: solid red;
border-left: solid red
}
Since, to some extent, the properties have overlapping
functionality, the order in which the rules are specified is
important.
Example(s):
Consider this example:
BLOCKQUOTE {
border-color: red;
border-left: double;
color: black
}
In the above example, the color of the left border is black,
while the other borders are red. This is due to 'border-left' setting the
width, style, and color. Since the color value is not given by the
'border-left' property, it
will be taken from the 'color'
property. The fact that the 'color' property is set after the 'border-left' property is not
relevant.
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