8 Box model

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.

8.1 Box dimensions

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 is 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".

content edge or inner edge	surrounds the element's rendered content.
padding edge	surrounds the box padding. If the padding has 0 width, the padding edge is the same as the content edge. of a box defines the edges of the containing block established by the box.
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	surrounds the box margin. If the margin has 0 width, the margin edge is the same as the border edge.

Each edge is 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.

8.2 Example of margins, padding, and borders

<STYLE type="text/css">                N.B. px or em... is required 	(6/22/08 ie doesn't seem to like list)
UL {  background: green;     margin:  8px 60px 4px 20px;    /* top,right, bottom, left   */
                             padding: 3px 20px 1px 5px;     /* No borders set */     }
LI {  color:red;                  /* text color is red */ 
      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-color: black;  border-style: dashed;  border-width: medium;  }   /* sets border width on all sides */
</STYLE>
<UL>
      <LI>First element of list
      <LI class="withborder">Second element of list is longer  to illustrate wrapping.
</UL>

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.

• 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).

8.3 Margin properties

Margin properties specify the width of the margin area of a box.
The margin shorthand property sets the margin for all four sides.

The properties defined in this section refer to the <margin-width> value type:

<length>	Specifies a fixed width. relative units are: i.fem: font-size ex: x-height px: pixels,
<percentage>	The percentage is calculated with respect to the width of the generated boxs 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 computing widths and margins for behavior. Negative values for margin properties are allowed, but there may be implementation-specific limits. margin-top, margin-right, margin-bottom, margin-left

Value:<margin-width> | inherit Initial:0, Applies to all elements, Not Inherited, Percentages:refers to width of containing block

These properties set the top, right, bottom, and left margin of a box.

H1 { margin-top: 2em }

margin Value: <margin-width>{1,4} | inherit; Initial: not defined for shorthand properties; Applies to: all elements; not Inherited ; Percentages: refer to width of containing block

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.

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) */
}

8.3.1 Collapsing margins

In this specification, the expression collapsing margins means that adjoining margins (no padding or border areas separate them) of two or more boxes (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.

See examples of margin, padding, and borders for an illustration of collapsed margins.

8.4 Padding properties

padding-top, padding-right, padding-bottom, padding-left, and padding

The padding properties specify the width of the padding area of a box. The padding shorthand property sets the padding for all four sides

The properties defined in this section refer to the <padding-width> value type: <length> <percentage> The percentage is calculated with respect to the width of the generated box's containing block, even for padding-top and padding-bottom.

Values for padding values cannot be negative.
Values for padding properties refer to the width of the generated box's containing block.

Value:<padding-width> | inherit Initial:0; Applies to:all elements ; Not inherited Percentages:refer to width of containing block

These properties set the top, right, bottom, and left padding of a box.

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

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 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 i.e:

1		1		1		padding: all
2    2		2    2		4    2		padding top+bot left+right
1		3		3		padding: top left+right bot
						padding: top right bot left

The surface color or image of the padding area is specified via the background property:

H1 {   background:pink;   padding: 4em  8em  16em; } 

<h1>eXaMple</h1>

eXaMple

(See margins)

<img src=gifs/Garfield.gif  style= background:pink;   padding: 8em 16em 32em; >

The example above specifies a 4em padding-top and an 8em horizontal padding padding-right + padding-left and a 16em padding-bottom) .

The 'em' unit is relative to the element's font size: '1em' is equal to the size of the font in use.

8.5 Border properties

The border properties specify the width, color, and style of the border area of a box.
These properties apply to all elements.
HTML user agents may render borders for certain elements (e.g., buttons, menus, etc.) differently than for "ordinary" elements.

8.5.1 Border width:

The border width properties specify the width of the border area. thin , medium or thick
<length> The border's thickness has an explicit positive value.

The interpretation of thin, medium, thick depends on the user agent.
Thin <= medium <= thick. (dugh!) and must be constant throughout a document.

Value:<border-width>| inherit

Initial:medium; Applies to:all elements ; Not inherited

• only one value, it applies to all sides.
• two values, the top and bottom borders are set to the first value and the right and left are set to the second.
• 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.

H1 { border-width: thin }                   /* thin thin thin thin */

IE doesn't do it
H1 { border-width: thin thick }             /* thin thick thin thick */
H1 { border-width: thin thick medium }      /* thin thick medium thick */

8.5.2 Border color:

The border color properties specify the color of a box's border.

Value:<color> | inherit

Initial:the value of the 'color' property; Applies to:all elements ; Not inherited

The border-color property sets the color of the four borders.

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.

In this example, the border will be a solid black line.

P { 
  color: black; 
  background: white; 
  border: solid;
}

8.5.3 Border style: border-top-style, border-right-style, border-bottom-style, border-left-style, and border-style

<img src=gifs/Garfield.gif style=border-width:3;>.

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,

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	a series of dots.
dashed	a series of short line segments.
solid	a single line segment.
double	two solid lines. The sum of the two lines and the space between them equals the value of border-width (i.e. border-width must be greater than 3) <img src=gifs/Garfield.gif style=border-width:3;border-style:double;> .
groove	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	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 boxs background.
The color of borders drawn for values of groove, ridge, inset, and outset depends on the elements color property.

Conforming HTML user agents may interpret dotted, dashed, double, groove, ridge, inset, and outset to be solid.

P { border: solid red }
P {  border-top: solid red;  border-right: solid red;  border-bottom: solid red;  border-left: solid red}