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.
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 top, right,
bottom, and left 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:
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 top, right, bottom, and left
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 background style of the content, padding, and border areas of a
box is specified by the 'background' property of the
generating element. Margin backgrounds 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: yellow;
margin: 12px 12px 12px 12px;
padding: 3px 3px 3px 3px;
/* No borders set */
}
LI {
color: white; /* text color is white */
background: blue; /* Content, padding will be blue */
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: lime;
}
</STYLE>
</HEAD>
<BODY>
<UL>
<LI>First element of list
<LI class="withborder">Second element of list is
a bit 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. (Image is not to scale.)
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 margin edge height of each LI box depends on 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 (yellow) 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. These properties apply to all elements, but
vertical margins will not have any effect on non-replaced inline
elements.
The properties defined in this section refer to the <margin-width>
value type, which may take one of the following values:
The percentage is calculated
with respect to the width of the generated box's
containing block.
Note that this is true for 'margin-top' and 'margin-bottom' as well.
If the containing block's width depends on this element, then the
resulting layout is undefined in CSS 2.1.
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 non-empty content, padding or border areas
or clearance separate them) of
two or more boxes (which may be next to one another or nested) combine
to form a single margin.
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 maximum of the absolute values
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.
Note. Adjoining boxes may be generated by
elements that are not related as siblings or ancestors.
Vertical margins between a floated box and any other box do not
collapse (not even between a float and its in-flow children).
Vertical margins of elements with 'overflow' other than 'visible' do
not collapse with their in-flow children.
Margins of absolutely
positioned boxes do not collapse (not even with their
in-flow children).
Margins of inline-block elements
do not collapse (not even with their in-flow children).
If the top and bottom margins of a box are adjoining, then it is
possible for margins to collapse through it. In this case, the
position of the element depends on its relationship with the other
elements whose margins are being collapsed.
If the element's margins are collapsed with its parent's top
margin, the top border edge of the box is defined to be the same as
the parent's.
Otherwise, either the element's parent is not taking part in the
margin collapsing, or only the parent's bottom margin is involved. The
position of the element's top border edge is the same as it would have
been if the element had a non-zero top border.
An element that has had clearance applied to it never
collapses its top margin with its parent block's bottom margin.
Note that the positions of elements that have been collapsed
through have no effect on the positions of the other elements with
whose margins they are being collapsed; the top border edge position
is only required for laying out descendants of these
elements.
Margins of the root element's box do not collapse.
The bottom margin of an in-flow block-level element is always
adjoining to the top margin of its next in-flow block-level
sibling, unless that sibling has clearance.
The top margin of an in-flow block-level element is adjoining to
its first in-flow block-level child's top margin if the element has no
top border, no top padding, and the child has no clearance.
The bottom margin of an in-flow block-level element with a 'height' of 'auto' and 'min-height' less than the
element's used height is adjoining to its last in-flow block-level
child's bottom margin if the element has no bottom padding or
border.
An element's own margins are adjoining if the 'min-height' property is zero, and
it has neither vertical borders nor vertical padding, and it has a
'height' of either 0 or 'auto',
and it does not contain a line box, and all of its in-flow children's
margins (if any) are adjoining.
When an element's own margins collapse, and that element has had
clearance applied to it, its top margin collapses with the adjoining
margins of subsequent siblings but that resulting margin does not
collapse with the bottom margin of the parent block.
Collapsing is based on the used value of 'padding', 'margin', and 'border' (i.e., after resolving any
percentages). The collapsed margin is
calculated over the used value of the various 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:
The percentage is calculated with
respect to the width of the generated box's containing block, even for
'padding-top' and 'padding-bottom'.
If the containing block's width depends on this element, then the
resulting layout is undefined in CSS 2.1.
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.
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 user interface 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:
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:
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 may take one of the following values:
The opposite of 'inset': the
border makes the 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 border
color properties, but UAs may choose their own algorithm to
calculate the actual colors used. For instance, if the 'border-color'
has the value 'silver', then a UA could use a gradient of colors from
white to dark gray to indicate a sloping border.
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:
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.
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.
For each line box, UAs must take the inline boxes generated for
each element and render the margins, borders and padding in visual
order (not logical order).
When the element's 'direction' property is 'ltr', the
left-most generated box of the first line box in which the element
appears has the left margin, left border and left padding, and the
right-most generated box of the last line box in which the element
appears has the right padding, right border and right margin.
When the element's 'direction' property is 'rtl', the
right-most generated box of the first line box in which the element
appears has the right padding, right border and right margin, and the
left-most generated box of the last line box in which the element
appears has the left margin, left border and left padding.
