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The CSS box model describes the rectangular boxes that aregenerated for elements in thedocumenttree and laid out according to thevisual formattingmodel.
Each box has acontent area (e.g.,text, an image, etc.) and optional surroundingpadding,border, andmargin areas; the sizeof each area is specified by properties defined below. The followingdiagram shows how these areas relate and the terminology used to referto 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:
Each edge may be broken down into a top, right, bottom, and leftedge.
The dimensions of the content area of a box — the The background style of the content, padding, and border areas of abox is specified by the This example illustrates how margins, padding, and bordersinteract. The example HTML document: results in adocument tree with(among other relationships) a UL element that has two LIchildren. The first of the following diagrams illustrates what this examplewould produce. The second illustrates the relationship between themargins, padding, and borders of the UL elements and those of itschildren LI elements. (Image is not to scale.) Note that: Margin properties specify the width of themargin area of a box. The The properties defined in this section refer to the Negative values for margin properties are allowed, but there may beimplementation-specific limits. These properties have no effect on non-replaced inlineelements. These properties set the top, right, bottom, and left margin of abox. Example(s): The'margin' property is ashorthand property for setting If there is only one component value, it applies to allsides. If there are two values, the top and bottom marginsare set to the first value and the right and left margins areset to the second. If there are three values, the top isset to the first value, the left and right are set to thesecond, and the bottom is set to the third. If there arefour values, they apply to the top, right, bottom, and left,respectively. Example(s): The last rule of the example above is equivalent to the examplebelow: In CSS, the adjoining margins of two or more boxes (which might ormight not be siblings) can combine to form a single margin. Marginsthat combine this way are said to Adjoining vertical margins collapse, except: Horizontal margins never collapse. Two margins are A collapsed margin is considered adjoining to another margin if anyof its component margins is adjoining to that margin. Note. Adjoining margins can begenerated by elements that are not related as siblings or ancestors. Note the above rules imply that: When two or more margins collapse, the resulting margin width isthe maximum of the collapsing margins' widths. In the case of negativemargins, the maximum of the absolute values of the negative adjoiningmargins is deducted from the maximum of the positive adjoiningmargins. If there are no positive margins, the maximum of theabsolute values of the adjoining margins is deducted from zero. If the top and bottom margins of a box areadjoining, then it is possible for margins to Note that the positions of elements that have been collapsedthrough have no effect on the positions of the other elements withwhose margins they are being collapsed; the top border edge positionis only required for laying out descendants of these elements. The padding properties specify the width of thepadding area of a box. The The properties defined in this section refer to the Unlike margin properties, values for padding values cannot benegative. Like margin properties, percentage values for paddingproperties refer to the width of the generated box's containing block. These properties set the top, right, bottom, and left padding ofa box. Example(s): The'padding' property is ashorthand property for setting If there is only one component value, it applies to allsides. If there are two values, the top and bottom paddingsare set to the first value and the right and left paddings areset to the second. If there are three values, the top isset to the first value, the left and right are set to thesecond, and the bottom is set to the third. If there arefour values, they apply to the top, right, bottom, and left,respectively. The surface color or image of the padding area is specified viathe'background' property: Example(s): The example above specifies a '1em' vertical padding ( The border properties specify the width, color, and style of theborder area of a box. These propertiesapply 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 theborder area. The propertiesdefined in this section refer to the The interpretation of the first three values depends on the useragent. 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' atthe same place in the style sheet. If there is only one component value, it applies to allsides. If there are two values, the top and bottom bordersare set to the first value and the right and left areset to the second. If there are three values, the top isset to the first value, the left and right are set to thesecond, and the bottom is set to the third. If there arefour values, they apply to the top, right, bottom, and left,respectively. Example(s): In the examples below, the comments indicate the resulting widthsof the top, right, bottom, and left borders: 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 followingmeanings: The'border-color'property can have from one to four component values, and the valuesare set on the different sides as for If an element's border color is not specified with a border property, user agents must use the valueof the element's Example(s): In this example, the border will be a solid black line. The border style properties specify the line style of a box'sborder (solid, double, dashed, etc.). The properties defined in thissection refer to the All borders are drawn on top of the box's background. The color ofborders drawn for values of 'groove', 'ridge', 'inset', and 'outset'depends on the element'sbordercolor properties, but UAs may choose their own algorithm tocalculate the actual colors used. For instance, if the 'border-color'has the value 'silver', then a UA could use a gradient of colors fromwhite to dark gray to indicate a sloping border. The'border-style'property sets the style of the four borders. It can have from one tofour component values, and the values are set on the different sides as for'border-width' above. Example(s): In the above example, the horizontal borders will be 'solid' andthe vertical borders will be 'dotted'. Since the initial value of the border styles is 'none', no borderswill be visible unless the border style is set. This is a shorthand property for setting the width, style, andcolor of the top, right, bottom, and left border of a box. Example(s): The above rule will set the width, style, and color of the borderbelow the H1 element. Omitted values are set totheirinitial values. Sincethe following rule does not specify a border color, the border willhave the color specified by the'color' property: The'border' property is ashorthand property for setting the same width, color, and style forall four borders of a box. Unlike the shorthand Example(s): For example, the first rule below isequivalent to the set of four rules shown after it: Since, to some extent, the properties have overlappingfunctionality, the order in which the rules are specified isimportant. Example(s): Consider this example: In the above example, the color of the left border is black,while the other borders are red. This is due to For each line box, UAs must take the inline boxes generated foreach element and render the margins, borders and padding in visualorder (not logical order). When the element's When the element's8.2Example of margins, padding, and borders
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//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>Second element of list is a bit longer to illustrate wrapping. </UL> </BODY></HTML>8.3Margin properties:'margin-top','margin-right','margin-bottom','margin-left', and'margin'
Value: <margin-width> |inherit Initial: 0 Applies to: all elements except elements with table display types other than table-caption, table and inline-table Inherited: no Percentages: refer to width of containing block Media: visual Computed value: the percentage as specified or the absolute length Value: <margin-width> |inherit Initial: 0 Applies to: all elements except elements with table display types other than table-caption, table and inline-table Inherited: no Percentages: refer to width of containing block Media: visual Computed value: the percentage as specified or the absolute length h1 { margin-top: 2em }Value: <margin-width>{1,4} |inherit Initial: see individual properties Applies to: all elements except elements with table display types other than table-caption, table and inline-table Inherited: no Percentages: refer to width of containing block Media: visual Computed value: see individual properties 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 */body { margin-top: 1em; margin-right: 2em; margin-bottom: 3em; margin-left: 2em; /* copied from opposite side (right) */}8.3.1Collapsing margins
8.4Padding properties:'padding-top','padding-right','padding-bottom','padding-left', and'padding'
Value: <padding-width> |inherit Initial: 0 Applies to: all elements except table-row-group, table-header-group, table-footer-group, table-row, table-column-group and table-column Inherited: no Percentages: refer to width of containing block Media: visual Computed value: the percentage as specified or the absolute length blockquote { padding-top: 0.3em }Value: <padding-width>{1,4} |inherit Initial: see individual properties Applies to: all elements except table-row-group, table-header-group, table-footer-group, table-row, table-column-group and table-column Inherited: no Percentages: refer to width of containing block Media: visual Computed value: see individual properties h1 { background: white; padding: 1em 2em;}8.5Border properties
8.5.1Border width:
Value: <border-width> |inherit Initial: medium Applies to: all elements Inherited: no Percentages: N/A Media: visual Computed value: absolute length; '0' if the border style is 'none' or 'hidden' Value: <border-width>{1,4} |inherit Initial: see individual properties Applies to: all elements Inherited: no Percentages: N/A Media: visual Computed value: see individual properties 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 */8.5.2Border color:'border-top-color','border-right-color','border-bottom-color','border-left-color', and'border-color'
Value: <color> | transparent |inherit Initial: the value of the 'color' property Applies to: all elements Inherited: no Percentages: N/A Media: visual Computed value: when taken from the 'color' property, the computed value of 'color'; otherwise, as specified Value: [<color> | transparent ]{1,4} |inherit Initial: see individual properties Applies to: all elements Inherited: no Percentages: N/A Media: visual Computed value: see individual properties p { color: black; background: white; border: solid;}8.5.3Border style:'border-top-style','border-right-style','border-bottom-style','border-left-style', and'border-style'
Value: <border-style> |inherit Initial: none Applies to: all elements Inherited: no Percentages: N/A Media: visual Computed value: as specified Value: <border-style>{1,4} |inherit Initial: see individual properties Applies to: all elements Inherited: no Percentages: N/A Media: visual Computed value: see individual properties #xy34 { border-style: solid dotted }8.5.4Border shorthand properties:'border-top','border-right','border-bottom','border-left', and'border'
Value: [<border-width> ||<border-style> ||<'border-top-color'> ] |inherit Initial: see individual properties Applies to: all elements Inherited: no Percentages: N/A Media: visual Computed value: see individual properties h1 { border-bottom: thick solid red }H1 { border-bottom: thick solid }Value: [<border-width> ||<border-style> ||<'border-top-color'> ] |inherit Initial: see individual properties Applies to: all elements Inherited: no Percentages: N/A Media: visual Computed value: see individual properties p { border: solid red }p { border-top: solid red; border-right: solid red; border-bottom: solid red; border-left: solid red}blockquote { border: solid red; border-left: double; color: black;}8.6The box model for inline elements in bidirectional context
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