CSS3 Backgrounds and Borders Module

Abstract

CSS (Cascading Style Sheets) is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc. This draft contains the proposed functionality for CSS level 3 to describe borders and backgrounds, including borders consisting of images and backgrounds with multiple images. It includes and extends the functionality of CSS level 2 [CSS2], which builds on CSS level 1 [CSS1].

This module replaces two earlier drafts: CSS3 Backgrounds and CSS3 Border.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

The (archived) public mailing list www-style@w3.org (see instructions) is preferred for discussion of this specification. When sending e-mail, please put the text “css3-background” in the subject, preferably like this: “[css3-background] …summary of comment…”

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced under the 24 January 2002 CPP as amended by the W3C Patent Policy Transition Procedure. The Working Group maintains a public list of patent disclosures relevant to this document; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) with respect to this specification should disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of contents

• 1. Dependencies with other CSS 3 Modules
• 2. Context
  • 2.1. Changes from CSS 2.1
• 3. The layout model of backgrounds and borders
• 4. The 'background-color' property
• 5. The 'background-image' property
• 6. The 'background-repeat' property
• 7. The 'background-attachment' property
• 8. The 'background-position' property
• 9. The 'background-clip' property
• 10. The 'background-origin' property
• 11. The 'background-size' property
• 12. The 'background-break' property
• 13. Layering multiple background images
• 14. The 'background' property
• 15. The 'border-color' properties
• 16. The 'border-style' properties
• 17. The 'border-width' properties
• 18. The 'border-image' property
• 19. The 'border-radius' properties
• 20. The 'border-break' property
• 21. The 'border-top', 'border-bottom', 'border-right', 'border-left', and 'border' properties
• 22. The 'box-shadow' property
• 23. The background of the canvas
• 24. Profiles
• 25. Conformance
• 26. Tests
• 27. Acknowledgments
• 28. CR exit criteria
• 29. References
  • Normative references
  • Other references
• Property index
• Index

1. Dependencies with other CSS 3 Modules

• General Syntax [CSS3SYN]
• Values and Units [CSS3VAL]
• Cascading and Inheritance [CSS3CASCADE]
• Box Model [CSS3BOX]
• Color module [CSS3COLOR]
• Absolute Positioning Module [CSS3POS] (in particular the section “Elaborate description of Stacking Contexts”)

2. Context

Members of the CSS Working Group proposed during the Clamart meeting to modularize the CSS specification.

This modularization, and the externalization of the general syntax, of CSS will reduce the size of the specification and allow new types of specifications to use selectors and/or CSS general syntax.

This specification will have its own test suite, with several tests per concept introduced in this document, as well as tests for multiple concepts used together. This test suite will not consist of full conformance tests, but is intended to ensure interoperability between implementations and will form part of the CR exit criteria.

2.1. Changes from CSS 2.1

[This section needs to be revised.]

'Background-clip', 'background-break', and 'background-origin' are new. Also there are changes to the 'background' shorthand property.

'Background-attachment' has a new value 'local'.

The background can now have multiple images. As a consequence, several properties now accept a comma-separated list of values.

3. The layout model of backgrounds and borders

(This section is informative.)

When elements are rendered according to the CSS box model [CSS3BOX], each element is either not displayed at all, or formatted as one or more rectangular boxes. Each box has a rectangular content area, a band of padding around the content, a border around the padding, and a margin outside the border. (The margin may actually be negative, but margins have no influence on the background and border, so that doesn't concern the model for borders and backgrounds.)

The child elements of an element usually create boxes of their own, that are placed inside the content area of their parent, although they may also be placed outside it.

The reason an element may result in more than one box, is that elements may be broken at the end of a line (for inline elements), at the end of a column or at the end of a page and create further boxes in the next line, column or page.

The properties of this module deal with the contents of the border area and the background of the content, padding and border areas.

That background may be fully transparent (the default), or it may be filled with a color, one or more images, or both. The background properties specify what color ('background-color') and images ('background-image') to use, where they are placed ('background-position'), whether they are repeated or scaled ('background-repeat', 'background-size'), etc.

The border can either be a predefined style (solid line, double line, dotted line, pseudo-3D border, etc.) or it can be an image. In the former case, three properties define the style of the four border sides ('border-style'), color ('border-color') and thickness ('border-width'). In the latter case, the sides and corners are taken from the sides and corners of an image specified with 'border-image'. The image can be sliced, scaled and stretched in various ways to fit the size of the border.

If an element is broken into multiple boxes, 'border-break' and 'background-break' define how the borders and background are divided over the various boxes.

There is also another kind of border-like decoration that can be added to an element: a drop shadow, via the 'box-shadow' property.

4. The 'background-color' property

This property sets the background color of an element. Valid color values are defined in the Color Module [CSS3COLOR].

h1 { background-color: #F00 }

5. The 'background-image' property

This property sets the background image(s) of an element, or 'none' for no image.

Images are drawn with the first specified one on top (closest to the user) and each subsequent image behind the previous one. The 'background-color' is painted below all the images.

See the section “Layering multiple background images” for how each image is tiled or stretched.

body { background-image: url("marble.svg") } 
p { background-image: none }
div { background-image: url(tl.png), url(tr.png) }

Implementations may optimize by not downloading and drawing images that are not visible (e.g., because they are behind other, fully opaque images).

An image that is empty (zero width or zero height), that fails to download, or that otherwise cannot be displayed (e.g., because it is not in a supported image format) has the same effect as a non-empty transparent image.

If 'background-repeat' or 'background-position' has more comma-separated values than 'background-image', the series of values is repeated as needed.

background-image: url(a), url(b);
background-position: top, right, bottom, left, center;
background-repeat: no-repeat, no-repeat;

background-image: url(a), url(b), url(a), url(b), url(a);
background-position: top, right, bottom, left, center;
background-repeat: no-repeat, no-repeat, no-repeat,
  no-repeat, no-repeat;

It seems more intuitive to let 'background-image' determine the number of layers and truncate or extend 'background-repeat' and 'background-position' accordingly. Also because only 'background-image' has the value 'none'.

Editor's Note: Conformance properties for an image should be addressed here: MIME type image/*, require support for PNG, refer to profiles…

6. The 'background-repeat' property

The value <repeat> stands for: repeat-x | repeat-y | [repeat | space | no-repeat]{1,2}.

See the section “Layering multiple background images” for how each comma-separated value is applied to each image of the 'background-image' property.

The tiling and positioning of the background image on inline elements is described under the 'background-break' property.

Should there also be values of "repeat-up", "repeat-down", "repeat-right", and "repeat-left" for this property?

Values have the following meanings:

repeat-x

Equivalent to 'repeat no-repeat'.

repeat-y

Equivalent to 'no-repeat repeat'

Otherwise, if there is one value, it sets both the horizontal and vertical repeat. If there are two, the first one is for the horizontal direction, the second for the vertical one. As follows:

repeat

The image is repeated in this direction as often as needed to cover the area. The image is clipped at the border or padding edge (depending on 'background-clip').

no-repeat

The image is not repeated in this direction. If it is too large, the image is clipped at the border or padding edge (depending on 'background-clip').

space

The image is repeated as often as it will fit without being clipped and then the images are spaced out to fill the area. The value of 'background-position' for this direction is ignored. Be more precise? Is the space at the edge the same as between the images or is there no space at the edge (unless there is only one image)? If the image is too big to fit, one image is placed in the center and clipped. Or: If the image is too big, it is positioned according to 'background-position'.

Unless one of the two keywords is 'no-repeat', the whole background will be tiled, i.e., not just one vertical strip and one horizontal strip.

body { 
  background: white url("pendant.png");
  background-repeat: repeat-y;
  background-position: center;
}
p {
  background-image: url(star.svg), url(pattern.png);
  background-repeat: no-repeat, repeat
}

7. The 'background-attachment' property

If background images are specified, this property specifies whether they are fixed with regard to the viewport ('fixed') or scroll along with the document ('scroll' and 'local').

See the section “Layering multiple background images” for how each comma-separated value is applied to each image of the 'background-image' property.

The difference between 'scroll' and 'local' is only visible when the element has a scrolling mechanism (see 'overflow'): in the case of 'scroll', the background does not scroll with the element's content. (It is attached to the element's border, as it were.) In the case of 'local', the background scrolls along with the element's content. In the this case, the background behind the element's border (if any) scrolls as well, even though the border itself does not scroll with the contents.

Even if the image is fixed, it is still only visible when it is in the content, padding, or border area of the element. Thus, unless the image is tiled ('background-repeat: repeat'), it may be invisible.

body { 
  background: red url("pendant.gif");
  background-repeat: repeat-y;
  background-attachment: fixed;
}

User agents that do not support 'fixed' backgrounds (for example due to limitations of the hardware platform) should ignore declarations with the keyword 'fixed'. For example:

body {
   /* For all UAs: */
   background: white url(paper.png) scroll;
   /* For UAs that do fixed backgrounds: */
   background: white url(ledger.png) fixed;
}
h1 {
   /* For all UAs: */
   background: silver;
   /* For UAs that do fixed backgrounds: */
   background: url(stripe.png) fixed, white url(ledger.png) fixed;
}

8. The 'background-position' property

The <bg-position> stands for: [ [ <percentage> | <length> | left | center | right ] [ <percentage> | <length> | top | center | bottom ]? ] | [ [ left | center | right ] || [ top | center | bottom ] ]

If a background image has been specified, this property specifies its initial position. The section “Layering multiple background images” defines to which image of 'background-image' each of the comma-separated values applies.

<percentage> <percentage>

With a value pair of '0% 0%', the upper left corner of the image is aligned with the upper left corner of some area, usually the box's padding edge, but possibly the border edge, content edge or the viewport (see below). A value pair of '100% 100%' places the lower right corner of the image in the lower right corner of the area. With a value pair of '14% 84%', the point 14% across and 84% down the image is to be placed at the point 14% across and 84% down the area.

<length> <length>

With a value pair of '2cm 1cm', the upper left corner of the image is placed 2cm to the right and 1cm below the upper left corner of the area.

top left,

left top

Same as '0% 0%'.

top,

top center,

center top

Same as '50% 0%'.

right top,

top right

Same as '100% 0%'.

left,

left center,

center left

Same as '0% 50%'.

center

center center

Same as '50% 50%'.

right,

right center,

center right

Same as '100% 50%'.

bottom left

left bottom

Same as '0% 100%'.

bottom,

bottom center,

center bottom

Same as '50% 100%'.

bottom right

right bottom

Same as '100% 100%'.

If only one percentage or length value is given, it sets the horizontal position only, and the vertical position will be 50%. If two values are given, the horizontal position comes first. Combinations of keyword, length and percentage values are allowed, (e.g., '50% 2cm' or 'center 2cm' or 'center 10%'). For combinations of keyword and non-keyword values, 'left' and 'right' may only be used as the first value, and 'top' and 'bottom' may only be used as the second value. Negative positions are allowed.

body { background: url("banner.jpeg") right top }    /* 100%   0% */
body { background: url("banner.jpeg") top center }   /*  50%   0% */
body { background: url("banner.jpeg") center }       /*  50%  50% */
body { background: url("banner.jpeg") bottom }       /*  50% 100% */

If the background image is fixed within the viewport (see the 'background-attachment' property), the image is placed relative to the viewport. Otherwise, it is placed relative to the element's padding or border area, depending on 'background-origin'.

body { 
  background-image: url("logo.png");
  background-attachment: fixed;
  background-position: 100% 100%;
  background-repeat: no-repeat;
} 

9. The 'background-clip' property

Determines whether the background extends into the border area or not. If the value is 'padding', the background is clipped to the padding edge and the background of the border is transparent. If the value is 'border', the background extends into the border area.

Each comma-separated value applies to one image of 'background-image'. See the section “Layering multiple background images” for how clip values and images are paired.

Editor's Note: Can the background color be placed under the border but not background images? Not currently.

Is it useful to allow separate clip values for each image, or is one value for the whole background enough?

10. The 'background-origin' property

Determines how the 'background-position' is calculated. With a value of 'padding', the position is relative to the padding edge ('0 0' is the upper left corner of the padding edge, '100% 100%' is the lower right corner). 'Border' means the position is relative to the border edge, and 'content' means relative to the content edge.

Each comma-separated value applies to one image of 'background-image'. See the section “Layering multiple background images” for how origin values and images are paired.

Note that if 'background-clip' is 'padding', 'background-origin' is 'border', and 'background-position' is 'top left' (the initial value), then the top left of the background image will be clipped.

11. The 'background-size' property

Is 'background-stretch' a better name?

Specifies the size of the background images. The section “Layering multiple background images” defines to which image of the 'background-image' property each of the comma-separated values applies.

If a comma-separated value has only one part (not counting the keyword 'round'), the second part is set to 'auto'. Of the two parts, the first one refers to the width, the second to the height of the corresponding background image. The addition of the keyword 'round' indicates that the width and height are approximate, as explained below. 'round' on its own is equivalent to 'auto auto round'.

The size of an image is established in two steps. The first step derives sizes as follows:

auto

The size that keeps the background image's original aspect ratio. Additionally, if the other part of the value is also 'auto', the image has its intrinsic size.

<length>

A specific size.

<percentage>

The percentage is relative to the width or height of the area given by 'background-origin'.

Negative values are not allowed. A size of zero is allowed, but causes the image not to be displayed. (The effect is the same as if it had been a transparent image.)

If the given size is accompanied by the keyword 'round', and the computed value of 'background-repeat' is repeat for the horizontal and/or vertical direction, there is a second step: The UA must reduce the width, resp., height so that the image fits a whole number of times in the background area. In the case of the width:

If X ≠ 0 is the width of the image (i.e., the specified length or percentage, or the intrinsic width if the 'background-size' is 'auto') and W is the width of the background area, then the rounded width X' = W / ceil(W / X)

The height is analogous. ceil() is a function that returns its argument if it is a whole number, otherwise the next bigger whole number.

If the width is reduced because of this formula, the aspect ratio is not retained, not even if the height was specifed as 'auto' and the vertical repeat as 'no-repeat'. Ditto if the height is reduced.

Should 'round' be specified for width and height separately? It then becoems possible to round in one direction and keep the aspect ratio in the other. E.g., instead of allowing the keyword in 'background-size', it could be one of the possible values in 'background-repeat': [ repeat | space | no-repeat | round ]{1,2}.

Is 'round' the right word? How about '~' in front of the number, or 'approx' or 'about'?

Is it better allow the size of the image to be rounded up as well as down?

div {
    background-image: url(plasma.png);
    background-size: 100%;
    background-origin: content}

p {
    background-image: url(tubes.png);
    background-size: 50% auto;
    background-origin: border}

para {
    background-size: 15px;
    background-image: url(tile.png)}

body {
    background-size: auto;
    background-image: url(flower.png)}

p {
    background-image: url(chain.png);
    background-repeat: repeat-y;
    background-size: 20% 30% round; }

12. The 'background-break' property

This property applies to inline flow elements (specifically, it does not apply to elements that are inline-block or inline-table), and to block-level elements when they are broken into several boxes (for example, across pages). The property does different things for different types of boxes.

For inline flow elements, values have the following meanings:

bounding-box

The background is drawn in a rectangle which is the smallest rectangle that encloses all inline boxes of the element. It is only visible within the padding (or border, depending on the 'background-clip' property) area of the inline boxes. If the boxes are on different pages, the pages are treated as one continuous page, glued together at their top and bottom (after stripping away the top and bottom page margins) and the rectangle is drawn on this continuous page. Still an issue if the pages aren't the same width, e.g., one is in landscape.

each-box (or discontinuous?)

The background is drawn in each inline box of the element. A no-repeat background image will thus be rendered once in each inline box of the element.

continuous

The effect is as if, after the element has been laid out (including any justification, bidi reordering, page breaks, etc.), all the element's boxes are taken and put one after the other in one long row; the background is applied to the bounding box of this row and then the boxes are put back, with their share of the background. The order in the row is the visual order of the boxes, not the logical order of the source. I.e., in a right-to-left containing block ('direction' is 'ltr'), the first box is the leftmost box on the first line and subsequent boxes are put to the right of it. In a right-to-left containing block, the first box is the rightmost on the first line and subsequent boxes are put to the left of it. Is this true? or the direction of the element itself?

For block-level elements, values have the following meanings: (EDITOR'S NOTE: following needs to be checked against what was in old "background-break (open|close)" property)

bounding-box

The background is tiled in a rectangle which is the smallest rectangle that encloses all boxes of the block-level element.

each-box

(or 'discontinuous'?) Each box of the block-level element is treated separately for background rendering.

continuous

For block-level elements, 'continuous' works identical to 'bounding-box'

13. Layering multiple background images

The properties 'background-image', 'background-origin', 'background-clip', 'background-repeat', 'background-size', and 'background-position' may have multiple comma-separated values. Excepting the case that 'background-image' is 'none', if the values are specified as follows:

backgound-image: w₁,…w_M
backgound-repeat: x₁,…x_R
backgound-size: y₁,…y_S
backgound-position: s₁,…s_P

the number of layers is N = max(M, R, S, P) [shouldn't it be M instead?].

Each of the properties is interpreted as if it had N values, by repeating the specified values like this:

backgound-image: w₁,…w_M, w₁,…w_M, w₁,… /* N values */
backgound-repeat: x₁,…x_R, x₁,…x_R, x₁,… /* N values */
backgound-size: y₁,…y_S, y₁,…y_S, y₁,… /* N values */
backgound-position: s₁,…r_P, s₁,…r_P, s₁,… /* N values */

background-image: url(flower.png), url(ball.png), url(grass.png);
background-position: center center, 20% 80%, top left;
background-origin: border, content;

background-image: url(flower.png), url(ball.png), url(grass1.png);
background-position: center center, 20% 80%, top left;
background-origin: border, content, border;

background-image: url(red.png), url(blue.png);
background-repeat: repeat-x, repeat-y, repeat-y;
background-position: 20% 25%, 40% 10%, 50% 15%, 70% 40%, 90% 35%;

background-image: url(red.png), url(blue.png), url(red.png),
    url(blue.png), url(red.png);
background-repeat: repeat-x, repeat-y, repeat-y, repeat-x, repeat-y;
background-position: 20% 25%, 40% 10%, 50% 15%, 70% 40%, 90% 35%;

There are other ways to add missing values: repeat the last value at the end, repeat the first value at the start, fill either at the end or at the start with the initial value…

Each of the images is repeated, sized, and positioned according to the corresponding value in the other properties. The first image in the list is the layer closest to the user, the next one is painted behind the first, and so on.

If 'background-image' is 'none', there are no layers (N = 0).

14. The 'background' property

<bg-layer> stands for: <'background-image'> && [ ( <'background-size'> ) ]? && <'background-repeat'>? && <'background-position'>? && <'background-attachment'>? && [ <'background-clip'> <'background-origin'>? ]?

<final-bg-layer> stands for: <'background-image'> || ( <'background-size'> ) || <'background-repeat'> || <'background-position'> || <'background-attachment'> || [ <'background-clip'> <'background-origin'>? ] || <'background-color'>

Note that <bg-layer> requires a <uri> for an image, while an image is optional in <final-bg-layer>; a color and attachment are permitted in <final-bg-layer>, but not in <bg-layer>.

The 'background' property is a shorthand property for setting most background properties at the same place in the style sheet. It sets each of 'background-color', 'background-position', 'background-size', 'background-repeat', 'background-clip', 'background-origin', 'background-attachment' and 'background-image' to the value given, or to its initial value, if no explicit value was specified. (It does not set 'background-break'.)

Note that the ::first-line pseudo-element is like an inline-level element for the purposes of the background (see section 5.12.1 of [CSS21]). That means, e.g., that in a left-justified first line, the background does not necessarily extend all the way to the right margin.

body { background: red }
p { background: url("chess.png") (10em round) gray
       40% repeat fixed border border}

body {
    background-color: red;
    background-position: 0% 0%;
    background-size: 30% 30%;
    background-repeat: repeat repeat;
    background-clip: border;
    background-origin: padding;
    background-attachment: scroll;
    background-image: none }

p {
    background-color: gray;
    background-position: 40% 50%;
    background-size: 10em 10em round;
    background-repeat: repeat repeat;
    background-clip: border;
    background-origin: border;
    background-attachment: fixed;
    background-image: url(chess.png) }

E { background: #CCC url("metal.jpg") (100% auto) no-repeat top left }

div { background: padding url(paper.jpg) white center }
div {
    background-color: white;
    background-image: url(paper.jpg);
    background-repeat: repeat;
    background-attachment: scroll;
    background-position: center;
    background-clip: padding;
    background-origin: padding;
    background-size: auto auto }

background: url(a.png) top left no-repeat,
  url(b.png) center (100% 100%) no-repeat, url(c.png) white

background-image:      url(a.png), url(b.png),        url(c.png);
background-position:   top left,   center,            top left;
background-repeat:     no-repeat,  stretch no-repeat, repeat;
background-clip:       border,     border,            border;
background-origin:     padding,    padding,           padding;
background-size:       auto auto,  100% 100%,         auto auto;
background-attachment: scroll,     scroll,            scroll;
background-color:      white;

Note that 'background: url(foo) white' and 'background: url(foo), white' have the same effect.

Having both clip and origin in the shorthand may be confusing, since they use the same keywords and their order is thus important. Maybe a simplification (due to Fantasai) is to allow '[border | padding | content]?' only once and use it to set clip and origin to the same value (or to their defaults, if absent).

15. The 'border-color' properties

These properties set the foreground color of the border specified by the border-style properties.

'Border-color' is a shorthand for the four 'border-*-color' properties. The four values set the top, right, bottom and left border, respectively. A missing left is the same as right, a missing bottom is the same as top, and a missing right is also the same as top.

16. The 'border-style' properties

These properties set the style of the border, unless there is a border image (see 'border-image').

'Border-style' is a shorthand for the other four. Its four values set the top, right, bottom and left border respectively. A missing left is the same as right, a missing bottom is the same as top, and a missing right is also the same as top.

<border-style> is: none | hidden | dotted | dashed | solid | double | dot-dash | dot-dot-dash | wave | groove | ridge | inset | outset. Values have the following meanings:

none

No border. Color and width are ignored (i.e., the border has width 0, unless the border is an image, see 'background-image'

hidden

Same as 'none', but has different behavior in the conflict resolution rules for tables [CSS3TBL].

dotted

A series of dots.

dashed

A series of dashes.

solid

A single line segment.

double

Two parallel solid lines with some space between them. (The thickness of the lines is not specified, but the sum of the lines and the space must equal 'border-width'.)

dot-dash

Alternating dots and dashes.

dot-dot-dash

Two dots and a dash.

wave

A wavy line.

groove

Looks as if it were carved in the canvas. (This is typically achieved by creating a “shadow” from two colors that are slightly lighter and darker than the 'border-color'.)

ridge

Looks as if it were coming out of the canvas.

inset

Looks as if the content on inside of the border is sunken into the canvas. Different meaning on table[ref] elements.

outset

Looks as if the content on the inside of the border is coming out of the canvas. Different meaning for table[ref] elements.

Borders are drawn in front of the element's background, but behind the element's content (in case it overlaps).

Note: There is no control over the spacing of the dots and dashes, nor over the length of the dashes. Implementations are encouraged to choose a spacing that makes the corners symmetrical.

Note: This specification does not define how borders of different styles should be joined in the corner. Also note that rounded corners may cause the corners and the contents to overlap, if the padding is less than the radius of the corner.

17. The 'border-width' properties

[Border-width doesn't allow percentages in CSS2; should we allow percentages (of the containing block's width) in CSS3?]

These properties set the thickness of the border.

The <border-width> is '<length> | <percentage> | thin | medium | thick'. The <length> and <percentage> may not be negative. The lengths corresponding to 'thin', 'medium' and 'thick' are not specified, but the values are constant throughout a document and thin ≤ medium ≤ thick. A UA could, e.g., make the thickness depend on the 'medium' font size: one choice might be 1, 3 & 5px when the 'medium' font size is 17px or less.

'Border-width' is a shorthand that sets the four 'border-*-width' properties. If it has four values, they set top, right, bottom and left in that order. If left is missing, it is the same as right; if bottom is missing, it is the same as top; if right is missing, it is the same as top.

Note that the initial width is 'medium', but the initial style is 'none' and therefore the used width is 0.

When the used width of the border is 0, we say that the border is absent.

18. The 'border-image' property

Specifies an image to use instead of the border styles given by the 'border-style' properties and an additional background image for the element. If the value is 'none' or if the image cannot be displayed, the border styles will be used.

The four numbers or percentages immediately following the <uri> specify which part of that image is used for which part of the border. They divide the image into nine parts: four corners, four edges and a middle part. The middle part is used as an extra background image.

The first of the four values is the height of the top edge and of the two top corners. The second is the width of the right edge and the two right corners. The third is the height of the bottom edge and the two bottom corners. The fourth is the width of the left edge and the two left corners.

Percentages are relative to the size of the image. Numbers represent pixels in the image (if the image is a raster image) or CSS px units (if not). Negative values are not allowed and values bigger than the size of the image are interpreted as 100%.

The <number>s are slightly confusing. People may be tempted to write 5px instead of 5, since on most screens, 5px is the same size as 5 image pixels.

If the sum of the right and left widths is equal to or greater than the width of the image, the images for the top and bottom edge and the middle part are empty, which has the same effect as if a non-empty transparent image had been specified for those parts. Analogously for the top and bottom values.

If the slash is present in the property value, the one to four values after it are used for the width of the border instead of the 'border-width' properties (but only if the specified image can be displayed). The order of the values is the same as for 'border-width'.

At the end of the value, there may be up to two keywords, that specify how the images for the sides and the middle part are scaled and tiled. If the second is absent, it is assumed to be the same as the first. If both are absent, the effect is the same as 'stretch stretch'.

The nine images are scaled in two steps:

step 1

• The two images for the top and bottom edges are made as tall as the top and bottom borders, respectively, (either using 'border-width' or the values specified after the slash) and their width is scaled proportionally.

• The images for the left and right edge are made as wide as the left and right borders, respectively, and their height is scaled proportionally.

• The corner images are scaled to be as wide and as tall as the two borders they are part of.

• The middle part is not scaled.

step 2

• If the first keyword is 'stretch', the top, middle and bottom images are further scaled to be as wide as the element's padding box. The height is not changed any further.

• If the first keyword is 'round', the top, middle and bottom images are reduced in width, so that exactly a whole number of them fit in the width of the padding box, as follows:

  If X ≠ 0 is the width of the image and W is the width of the padding area, then the rounded width X' = W / ceil(W / X)

• The effects of 'stretch' and 'round' for the second keyword are analogous, acting on the height of the left, middle and right images.

The resulting images are placed centered in their respective parts of the border (the middle image is put in the middle of the padding area) and then tiled. All images are drawn behind the element's content, in front of the element's background.

DIV {
  border-image: url("border.png") 27 27 27 27 round stretch;
  border: double orange 1em }

The 'border-image' property does not apply to table elements in a table with 'border-collapse' set to 'collapse'.

19. The 'border-radius' properties

The two length values of the 'border-radius' properties define the radii of a quarter ellipse that defines the shape of the corner (see the diagram below). The first value is the horizontal radius (or vertical if the 'writing-mode' is vertical). If the second length is omitted it is equal to the first (and the corner is thus a quarter circle). If either length is zero, the corner is square, not rounded. The border radius also causes the element's background to be rounded, even if the border is 'none'. Negative values are not allowed.

All border styles ('solid', 'dotted', 'inset', etc.) follow the curve of the border. Border images specified with 'border-image', however, are clipped at the outer edge of the curve. Or are they not affected at all?

20. The 'border-break' property

When a box that has a border is broken at a page break, column break, or, for inline elements, at a line break, a border and some padding can be inserted at the break, or the border can be left open.

If the style is set to 'none', no border and no padding are inserted.

Otherwise, padding is added as wide as the corresponding side of the 'padding' property and a border is added. If borders on that side would get an image, the image is used, scaled to the given width, otherwise the border gets the given style and color.

Note that, unlike for the other borders of the element, a value of 'none' suppresses any image border as well. If you want an image or nothing, use 'hidden' instead.

Is this asymmetry in the meaning of 'none' a problem? Another possibility is to have a simple on/off switch for the border at the break: 'border-break: show | hide'.

21. The 'border-top', 'border-bottom', 'border-right', 'border-left', and 'border' properties

This is a shorthand property for setting the width, style, and color of the top, right, bottom, and left border of a box. Omitted values are set to their initial values.

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.

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.

blockquote {
  border-color: red;
  border-left: double;
  color: black
}

22. The 'box-shadow' property

Multiple shadows may be overkill and hard to implement. What are the use cases?

One or more drop-shadows can be attached to a box. They are drawn just outside the border edge. The property is a comma-separated list of shadows, each specified by 3 length values and a color. Omitted lengths are 0, omitted colors are equal to the computed value of the 'color' property.

Shadows do not influence the layout: they may overlap with other boxes. Like backgrounds and borders, if they overlap other boxes, they are drawn behind any text or replaced element. In terms of the stacking context, the shadow of an element is drawn immediately below the background of that element.

The 3 lengths and the color of each shadow are interpreted as follows:

• The first length is the horizontal offset of the shadow. A positive value draws a shadow that is offset to the right of the box, a negative length to the left.
• The second length is the vertical offset. A positive value offsets the shadow down, a negative one up.
• The third length is a blur radius. Negative values are not allowed. If it is 0, the shadow is sharp, otherwise the larger the value, the more the shadow is blurred. The exact algorithm is not specified.
• The color is the color of the shadow.

If an element has multiple boxes, all of them get drop shadows, but shadows are only drawn where borders would also be drawn, see 'border-break'. If the box has a non-zero border-radius', the shadow is rounded in the same way.

span {border: thin solid; box-shadow: 0.2em 0.2em #CCC}

This example shows a shadow on the bottom right only, even though the box is transparent. Shouldn't we see a “real” shadow, projecting exactly the opaque parts of the box? What if the opaque parts are actually semi-opaque? Will the shadow be less intense there?

23. The background of the canvas

The background of the root element becomes the background of the canvas and extends to cover the entire canvas, although any images are positioned and stretched relative to the root element as if they were painted for that element alone. If the root's background-color value is 'transparent', the color is UA dependent. The root element does not paint this background again.

For HTML documents, however, it is recommended that authors specify the background for the BODY element rather than the HTML element. User agents should observe the following precedence rules to fill in the background of the canvas of HTML documents: if the value of any of the 'background' properties on the HTML element is different from its initial value, then use the background of the HTML element, else use the background of the BODY element. This does not apply to XHTML documents.

<!DOCTYPE html PUBLIC '-//W3C//DTD HTML 4.0//EN'
  >
<html>
  <head>
    <title>Setting the canvas background</title>
    <style type="text/css">
       body { background: url("http://example.org/marble.png") }
    </style>
  </head>
  <body>
    <p>My background is marble.</p>
  </body>
</html>

24. Profiles

(NOTE: Check profiles because of changes made!)

CSS1 Profile:

• 'background-color'
• 'background-image'
• 'background-repeat': only repeat, repeat-x, repeat-y, no-repeat
• 'background-attachment'
• 'background-position'
• 'background': only color, image, repeat, attachment and position
• 'background-origin' always has value content
• 'background-clip' always has value border
• 'background-break' always has value continuous

CSS2 Profile:

• 'background-color'
• 'background-image'
• 'background-repeat': only repeat, repeat-x, repeat-y, no-repeat
• 'background-attachment'
• 'background-position'
• 'background': only color, image, repeat, attachment and position
• 'background-origin' always has value padding
• 'background-clip' always has value border
• 'background-break' always has value continuous

Mobile and TV Profiles (informative, see their specifications):

• 'background-color'
• 'background-image'
• 'background-repeat': only repeat, repeat-x, repeat-y, no-repeat
• 'background-position'
• 'background': only color, image, repeat and position
• 'background-attachment': always has value scroll
• 'background-origin' always has value padding
• 'background-clip' always has value border
• 'background-break' always has value continuous

CSS3 Profile:

• Everything (as described in this specification)

25. Conformance

This section will define conformance with the present specification only.

The inability of a user agent to implement part of this specification due to the limitations of a particular device does not imply non-conformance.

User agents must observe the rules for handling parsing errors.

26. Tests

This specification will contain a test suite allowing user agents to verify their basic conformance to the specification. This test suite does not pretend to be exhaustive and does not cover all possible combined cases of W3C background functionality.

27. Acknowledgments

Tapas Roy was editor of the Border Module, before it was merged with the Background Module.

A set of properties for border images was initially proposed by fantasai. The current simplification (one image cut into nine parts) is due to Ian Hickson. (Though the original idea seems to originate with some anonymous Microsoft engineers.)

28. CR exit criteria

As described in the W3C process document, a Candidate Recommendation (CR) is a specification that W3C recommends for use on the Web. The next stage is “Recommendation,” when the specification is sufficiently implemented.

For this specification to be proposed as a W3C Recommendation, the following conditions shall be met:

1. There must be at least two interoperable implementations for every feature in the Module. For the purposes of this criterion, we define the following terms:

   feature

   a section or subsection of the specification

   interoperable

   passing the respective test case(s) in the CSS test suite, or, if the implementation is not a web browser, an equivalent test. Every relevant test in the test suite should have an equivalent test created if such a UA is to be used to claim interoperability. In addition if such a UA is to be used to claim interoperability, then there must one or more additional UAs which can also pass those equivalent tests in the same way for the purpose of interoperability. The equivalent tests must be made publicly available for the purposes of peer review.

   implementation

   a user agent which:

   1. implements the feature.
   2. is available (i.e. publicly downloadable or available through some other public point of sale mechanism). This is the "show me" requirement.
   3. is shipping (i.e. development, private or unofficial versions are insufficient).
   4. is not experimental (i.e. is intended for a wide audience and could be used on a daily basis.)

2. A minimum of sixth months of the CR period must have elapsed. This is to ensure that enough time is given for any remaining major errors to be caught.

3. Features will be dropped if two or more interoperable implementations are not found by the end of the CR period.

4. Features may/will also be dropped if adequate/sufficient (by judgment of CSS WG) tests have not been produced for those feature(s) by the end of the CR period.

29. References

Normative references

[CSS3BOX]

Bert Bos. CSS3 module: The box model. 24 October 2002. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2002/WD-css3-box-20021024

[CSS3CASCADE]

Håkon Wium Lie. CSS3 module: Cascading and inheritance. 19 February 2002. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2002/WD-css3-cascade-20020219

[CSS3COLOR]

Tantek Çelik; Chris Lilley. CSS3 Color Module. 14 May 2003. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2003/CR-css3-color-20030514

[CSS3SYN]

L. David Baron. CSS3 module: Syntax. 13 August 2003. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2003/WD-css3-syntax-20030813

[CSS3TEXT]

Michel Suignard. CSS3 Text Module. 14 May 2003. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2003/CR-css3-text-20030514

[CSS3VAL]

Håkon Wium Lie; Chris Lilley. CSS3 module: Values and Units. 13 July 2001. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2001/WD-css3-values-20010713

Other references

[CSS1]

Håkon Wium Lie; Bert Bos. Cascading Style Sheets (CSS1) Level 1 Specification. 17 December 1996. W3C Recommendation. Revised 11 January 1999. URL: http://www.w3.org/TR/1999/REC-CSS1-19990111

[CSS2]

Bert Bos; et al. Cascading Style Sheets, level 2 (CSS2) Specification. 12 May 1998. W3C Recommendation. URL: http://www.w3.org/TR/1998/REC-CSS2-19980512

[CSS21]

Bert Bos; et al. Cascading Style Sheets, level 2 revision 1. 25 February 2004. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2004/CR-CSS21-20040225

[CSS3POS]

Bert Bos. CSS3 Positioning Module. (forthcoming). W3C Working Draft. (Work in progress.)

[CSS3TBL]

Bert Bos; David Hyatt. CSS3 Tables Module. (forthcoming). W3C Working Draft. (Work in progress.)

Property index

The following properties are defined in other modules:

• 'border-bottom-width' 'border-left-width', 'border-right-width', 'border-top-width', 'border-width', see [CSS3BOX]
• 'color', see [CSS3COLOR]
• 'margin', 'padding', see [CSS3BOX]
• 'overflow', see [CSS3BOX]
• 'writing-mode', see [CSS3TEXT]
• 'z-index', see [CSS3POS]

The following data types are defined in [CSS3VAL]:

• <percentage>
• <length>
• <uri>
• <color>

Index

• background, #
• background-attachment, #
• background-break, #
• background-clip, #
• background-color, #
• background-image, #
• background-origin, #
• background-position, #
• background-repeat, #
• background-size, #
• border, #
• border-bottom, #
• border-bottom-color, #
• border-bottom-left-radius, #
• border-bottom-right-radius, #
• border-bottom-style, #
• border-bottom-width, #
• 'border-bottom-width', #
• border-break, #
• border-color, #
• border-image, #
• border-left, #
• border-left-color, #
• border-left-style, #
• border-left-width, #
• 'border-left-width', #
• border-radius, #
• border-right, #
• border-right-color, #
• border-right-style, #
• border-right-width, #
• 'border-right-width', #
• border-style, #
• border-style-none, #
• border-top, #
• border-top-color, #
• border-top-left-radius, #
• border-top-right-radius, #
• border-top-style, #
• border-top-width, #
• 'border-top-width', #
• border-width, #
• 'border-width', #
• box-shadow, #
• clip-border, #
• clip-padding, #
• 'color', #
• dashed, #
• dot-dash, #
• dot-dot-dash, #
• dotted, #
• double, #
• fixed, #
• groove, #
• hidden, #
• inset, #
• local, #
• <border-style>, #
• <color>, #
• <length>, #
• <percentage>, #
• <uri>, #
• 'margin', #
• no-repeat, #
• origin-border, #
• origin-content, #
• origin-padding, #
• outset, #
• 'overflow', #
• 'padding', #
• repeat, #
• ridge, #
• scroll, #
• solid, #
• space, #
• <bg-layer>, #
• <bg-position>, #
• <border-width>, #
• <final-bg-layer>, #
• <repeat>, #
• viewport, #
• wave, #
• 'writing-mode', #
• 'z-index', #