CSS Backgrounds Module Level 4

1. Introduction

When elements are rendered according to the CSS box model [CSS-BOX-3], 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.)

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

If an element is broken into multiple box fragments, box-decoration-break defines how the borders and background are divided over the various fragments. (An element can result in more than one fragment if it is broken at the end of a line, at the end of a column or at the end of a page; and continued in the next line, column or page.)

The relative stacking order of backgrounds, borders, and shadows is given in this module. For how these layers interact with other rendered content, see Appendix E “Elaborate description of Stacking Contexts” in [CSS2].

1.1. Module Interactions

This specification extends the parts related to backgrounds of CSS Backgrounds and Borders Module Level 3 [CSS3BG].

It provides specifications for the added background-repeat-* and `background-position-*' longhands, a new background-tbd property that allows to define the background layers excluding the color, and adds two new values to background-clip.

All properties in this module apply to the ::first-letter and ::first-line pseudo-elements.

1.2. Value Definitions

This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with other CSS modules may expand the definitions of these value types. For example, combining with CSS Images allows for using CSS gradients as background-image or border-image values. [CSS-IMAGES-3]

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords as their property value. For readability they have not been repeated explicitly.

2. Defining Backgrounds

Each box has a background layer that may be fully transparent (the default), or filled with a color and/or one or more images. The background properties specify what color (background-color) and images (background-image) to use, and how they are sized, positioned, tiled, etc.

The background properties are not inherited, but the parent box’s background will shine through by default because of the initial transparent value on background-color.

2.1. Base Color: the background-color property

This property sets the background color of a box. This color is drawn behind any background images.

h1 { background-color: #F00 } /* Sets background to red. */

The background color is clipped according to the background-clip value associated with the bottom-most background image layer.

2.2. Image Sources: the background-image property

This property specifies the background image(s) of an element. Images are drawn with the first specified one on top (closest to the user) and each subsequent image behind the previous one. The property’s value is given as a comma-separated list of <bg-image> values where

<bg-image> = <image> | none

A value of none counts as a background image layer but draws nothing. An image that is empty (zero width or zero height), that fails to download, or that cannot be displayed (e.g., because it is not in a supported image format) likewise counts as a layer but draws nothing.

See § 3 Layering Multiple Background Images for how background-image interacts with other comma-separated background properties to form each background image layer.

When setting a background image, authors should also specify a background-color that will preserve contrast with the text for when the image is unavailable.

For accessibility reasons, authors should not use background images as the sole method of conveying important information. See Web Content Accessibility Guideline F3 [WCAG20]. Images are not accessible in non-graphical presentations, and background images specifically might be turned off in high-contrast display modes.

Note: Stylistic foreground images can be provided in CSS with the content property. Semantically-important foreground images should be provided in the document markup, e.g. with the <img> tag in HTML.

Note: Media fragments can be used to display a portion of an image. The CSS Images module will provide fallback syntax for image formats and include additional controls for image display.

html { background-image: url("marble.svg") }
p { background-image: none }
div { background-image: url(tl.png), url(tr.png) }
main { background-image: radial-gradient(at bottom right, transparent, white); }

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

2.3. Tiling Images: the background-repeat-x, background-repeat-y, background-repeat-block, and background-repeat-inline properties

<repetition> = repeat | space | round | no-repeat

These properties specify whether and how background images are tiled along one axis after they have been sized and positioned.

repeat

The image is repeated in the given direction as often as needed to cover the background painting area.

space

The image is repeated in the given direction as often as will fit within the background positioning area without being clipped and then the repeated images are spaced out to fill the area. The first and last images touch the edges of the area. If the background painting area is larger than the background positioning area, then the pattern repeats to fill the background painting area. The value of background-position for this direction is ignored, unless there is not enough space for two copies of the image in this direction, in which case only one image is placed and background-position determines its position in this direction.

round

The image is repeated in the given direction as often as will fit within the background positioning area. If it doesn’t fit a whole number of times, it is rescaled so that it does. See the formula under background-size. If the background painting area is larger than the background positioning area, then the pattern repeats to fill the background painting area.

no-repeat

The image is placed once and not repeated in the given direction.

Unless one of the axes is set to no-repeat, the whole background painting area will be tiled, i.e., not just one vertical strip and one horizontal strip.

body {
  background: white url("pendant.png");
  background-repeat-y: repeat;
  background-position: center;
}

See the section § 3 Layering Multiple Background Images for how background-repeat-x, background-repeat-y, background-repeat-block, and background-repeat-inline interact with other comma-separated background properties to form each background image layer.

2.4. Tiling Images Shorthand: the background-repeat property

This shorthand sets the values for the background-repeat-x and background-repeat-y longhand properties.

<repeat-style> = repeat-x | repeat-y | <repetition>{1,2}

Single values for <repeat-style> have the following meanings:

repeat-x

Computes to repeat no-repeat.

repeat-y

Computes to no-repeat repeat.

repeat

Computes to repeat repeat.

space

Computes to space space

round

Computes to round round

no-repeat

Computes to no-repeat no-repeat

If a <repeat-style> value has two keywords, the first one is for the horizontal direction, the second for the vertical one.

body {
  background-image: url(dot.png) white;
  background-repeat: space;
}

See the section § 3 Layering Multiple Background Images for how background-repeat interacts with other comma-separated background properties to form each background image layer.

Should a 'background-repeat: extend' be added?

2.5. Affixing Images: 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 box (scroll) or its contents (local). The property’s value is given as a comma-separated list of <attachment> keywords where

<attachment> = scroll | fixed | local

fixed

The background is fixed with regard to the viewport. In paged media where there is no viewport, a fixed background is fixed with respect to the page box and therefore replicated on every page.

Note: There is only one viewport per view. Even if an box is a scroll container, a fixed background doesn’t move with the box.

local

The background is fixed with regard to the box’s contents: if the box has a scrolling mechanism, the background scrolls with the box’s contents, and the background painting area and background positioning area are relative to the scrollable overflow area of the box rather than to the border framing them. Because the scrollable overflow area does not include the border area, for scroll containers the border-box value of background-clip may be treated the same as padding-box.

scroll

The background is fixed with regard to the box itself and does not scroll with its contents. (It is effectively attached to the box’s border.)

Even if the image is fixed, it is still only visible when it is in the background painting area of the box or otherwise unclipped. (See § 4 Backgrounds of Special Elements for the cases when background images are not clipped.) Thus, unless the image is tiled, it may be invisible.

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

Note: User agents that do not support fixed backgrounds (for example due to limitations of the hardware platform) will 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;
}

See § 3 Layering Multiple Background Images for how background-attachment interacts with other comma-separated background properties to form each background image layer.

2.6. Background Positioning: the background-position-x, background-position-y, background-position-inline, and background-position-block properties

This section is still being worked out. The tricky thing is making all the start/end keywords work sanely.

This property specifies the background position’s horizontal component. An omitted origin keyword is assumed to be left.

This property specifies the background position’s vertical component. An omitted origin keyword is assumed to be top.

This property specifies the background position’s inline-axis component. An omitted origin keyword is assumed to be start.

This property specifies the background position’s block-axis component. An omitted origin keyword is assumed to be start.

<percentage>

A percentage for the horizontal offset is relative to (width of background positioning area - width of background image). A percentage for the vertical offset is relative to (height of background positioning area - height of background image), where the size of the image is the size given by background-size.

For example, with a value pair of 0% 0%, the upper left corner of the image is aligned with the upper left corner of, usually, the box’s padding edge. 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 75% 50%, the point 75% across and 50% down the image is to be placed at the point 75% across and 50% down the area.

<length>

A length value gives a fixed length as the offset. For example, 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 background positioning area.

top

Computes to 0% for the vertical position if one or two values are given, otherwise specifies the top edge as the origin for the next offset.

right

Computes to 100% for the horizontal position if one or two values are given, otherwise specifies the right edge as the origin for the next offset.

bottom

Computes to 100% for the vertical position if one or two values are given, otherwise specifies the bottom edge as the origin for the next offset.

left

Computes to 0% for the horizontal position if one or two values are given, otherwise specifies the left edge as the origin for the next offset.

center

Computes to 50% (left 50%) for the horizontal position if the horizontal position is not otherwise specified, or 50% (top 50%) for the vertical position if it is.

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% */

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

background-position: right 3em bottom 10px

See § 3 Layering Multiple Background Images for how background-position interacts with other comma-separated background properties to form each background image layer.

2.7. Background Positioning Shorthand: the background-position shorthand property

If background images have been specified, this property specifies their initial position (after any resizing) within their corresponding background positioning area.

This property is a shorthand property that sets background-position-x, background-position-y, background-position-block, and background-position-inline in a single declaration.

Its value is given as a comma-separated list of <bg-position> values, which are interpreted as <position> values with the resized background image as the alignment subject and the background positioning area as the alignment container.

Note: A pair of keywords can be reordered, while a combination of keyword and length or percentage cannot. So center left is valid while 50% left is not.

If three or four values are given, then each <length-percentage> represents an offset and must be preceded by a keyword, which specifies from which edge the offset is given. For example, background-position: bottom 10px right 20px represents a 10px vertical offset up from the bottom edge and a 20px horizontal offset leftward from the right edge. If three values are given, the missing offset is assumed to be zero.

Positive values represent an offset inward from the edge of the background positioning area. Negative values represent an offset outward from the edge of the background positioning area.

background-position: left 10px top 15px;   /* 10px, 15px */
background-position: left      top     ;   /*  0px,  0px */
background-position:      10px     15px;   /* 10px, 15px */
background-position: left          15px;   /*  0px, 15px */
background-position:      10px top     ;   /* 10px,  0px */
background-position: left      top 15px;   /*  0px, 15px */
background-position: left 10px top     ;   /* 10px,  0px */

<bg-position> =  <position> | <position-three>
<position-three> = [
  [ left | center | right ] && [ [ top | bottom ] <length-percentage> ]
|
  [ [ left | right ] <length-percentage> ] && [ top | center | bottom ]
]

The omitted <length-percentage> in the background-position-specific <position-three> syntax variant defaults to 0%.

Specify how the longhand properties are set. [Issue #9690]

2.7.1. Serialization of background-position values

The specified value and computed value of the <bg-position> type serialize exactly as defined in [CSS-VALUES-4] for <position>. For 3-value productions (which are not valid in <position>), the specified value serialization is identical to the equivalent 4-value syntax except that the omitted offset remains omitted.

2.8. Painting Area: the background-clip property

Determines the background painting area, which determines the area within which the background is painted. The syntax of the property is given with

<bg-clip> = <visual-box> | [ border-area || text ]

Or should this be defining the -webkit-background-clip property, saying that all the values are identical, with this additional text value?

<visual-box>

The background is painted within (clipped to) the specified box of the element.

text

The background is painted within (clipped to) the intersection of the border box and the geometry of the text in the element and its in-flow and floated descendants.

border-area

The background is clipped to the area painted by the border, taking border-width and border-style into account but ignoring any transparency introduced by border-color.

If both border-area and text are specified, the background is painted within (clipped to) the union of these two areas.

Note: The root element has a different background painting area and thus the background-clip property has no effect when specified on it. See § 4 Backgrounds of Special Elements.

Note: The background is always drawn behind the border, if any. See “Elaborate description of Stacking Contexts” in [CSS2] Appendix E.

See CSS Borders and Box Decorations 4 § 3.7 Corner Shaping: the corner-*-shape properties for how border-radius affects the shape of the background painting area.

See § 3 Layering Multiple Background Images for how background-clip interacts with other comma-separated background properties to form each background image layer.

2.9. Positioning Area: the background-origin property

This property determines the background positioning area: the area within which any background images are positioned. For elements rendered as multiple box fragments (e.g., inline boxes on several lines, boxes on several pages), specifies which boxes box-decoration-break [CSS-BREAK-3] operates on to determine the background positioning area(s).

padding-box

The position is relative to the padding box. (For single boxes 0 0 is the upper left corner of the padding edge, 100% 100% is the lower right corner.)

border-box

The position is relative to the border box.

content-box

The position is relative to the content box.

If the background-attachment value for this layer is fixed, then this property has no effect: in this case the background positioning area is the initial containing block.

Note: If background-clip is padding-box, background-origin is border-box, background-position is top left (the initial value), and the element has a non-zero border, then the top and left edges of the background image will be clipped.

See § 3 Layering Multiple Background Images for how background-origin interacts with other comma-separated background properties to form each background image layer.

2.10. Sizing Images: the background-size property

This property specifies the size of each background image. The property’s value is given as a comma-separated list of <bg-size> values where

<bg-size> = [ <length-percentage [0,∞]> | auto ]{1,2} | cover | contain

Values have the following meanings:

contain

Scale the image, while preserving its natural aspect ratio (if any), to the largest size such that both its width and its height can fit inside the background positioning area.

cover

Scale the image, while preserving its natural aspect ratio (if any), to the smallest size such that both its width and its height can completely cover the background positioning area.

[ <length-percentage [0,∞]> | auto ]{1,2}

The first value gives the width of the corresponding image, the second value its height. If only one value is given the second is assumed to be auto.

A <percentage> is relative to the background positioning area.

An auto value for one dimension is resolved by using the image’s natural aspect ratio and the size of the other dimension, or failing that, using the image’s natural size, or failing that, treating it as 100%.

If both values are auto then the natural width and/or height of the image should be used, if any, the missing dimension (if any) behaving as auto as described above. If the image has neither natural size, its size is determined as for contain.

Negative values are invalid.

div {
  background-image: url(plasma.png);
  background-repeat: no-repeat;
  background-size: 100% 100%;
  background-origin: content-box;
}

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

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

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

p {
  background-image: url(chain.png);
  background-repeat: no-repeat round;
  background-size: 20% 30%;
}

If background-repeat is round for one (or both) dimensions, there is a second step. The UA must scale the image in that dimension (or both dimensions) so that it fits a whole number of times in the background positioning area. In the case of the width (height is analogous):

If X ≠ 0 is the width of the image after step one and W is the width of the background positioning area, then the rounded width X' = W / round(W / X) where round() is a function that returns the nearest natural number (integer greater than zero).

If background-repeat is round for one dimension only and if background-size is auto for the other dimension, then there is a third step: that other dimension is scaled so that the original aspect ratio is restored.

div {
  background-image: url(image1.png);
  background-repeat: repeat;         /* default */
  background-size: auto;             /* default */
}

div {
  background-image: url(image2.png);
  background-repeat: repeat;         /* default */
  background-size: 3em;              /* = '3em auto' */
}

div {
  background-image: url(image3.png);
  background-repeat: round repeat;
  background-size: 3em auto;
}

div {
  background-image: url(image4.png);
  background-repeat: repeat round;
  background-size: 3em auto;
}

div {
  background-image: url(image5.png);
  background-repeat: round;
  background-size: auto 4em;
}

If the background image’s width or height resolves to zero, this causes the image not to be displayed. (The effect is the same as if it had been a transparent image.)

See § 3 Layering Multiple Background Images for how background-size interacts with other comma-separated background properties to form each background image layer.

2.11. Background Image Layers: the background-tbd shorthand property

The background-tbd property is a shorthand property that sets all the same properties as the background shorthand except for background-color, allowing authors to easily declare and position background images while letting background-color cascade through independently.

The name of this property is discussed in issue 9083.

p {
  background-color: green;
}

p {
  background-tbd:
    url(a.png) top left,
    url(b.png) top left no-repeat;
}

p {
  background: url(pass.png) green;   /* valid */
  background-tbd: url(fail.png) red; /* invalid */
}

2.12. Backgrounds Shorthand: the background property

The background property is a shorthand property for setting most background properties at the same place in the style sheet. The number of comma-separated items defines the number of background image layers. Given a valid declaration, for each layer the shorthand first sets the corresponding value of each of background-image, background-position, background-size, background-repeat, background-origin, background-clip and background-attachment to that property’s initial value, then assigns any explicit values specified for this layer in the declaration. Finally background-color is set to the specified color, if any, else set to its initial value.

This property’s value is given as a comma-separated list of values where

<bg-layer> = <bg-image> || <bg-position> [ / <bg-size> ]? || <repeat-style> || <attachment> || <visual-box> || <visual-box>
<final-bg-layer> =  <bg-image> || <bg-position> [ / <bg-size> ]? || <repeat-style> || <attachment> || <visual-box> || <visual-box> || <'background-color'>

Note: A color is permitted in <final-bg-layer>, but not in <bg-layer>.

If one <visual-box> value is present then it sets both background-origin and background-clip to that value. If two values are present, then the first sets background-origin and the second background-clip.

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

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

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

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

div {
  background: padding-box 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-box;
  background-origin: padding-box;
  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,   no-repeat,           repeat;
background-clip:       border-box,  border-box,          border-box;
background-origin:     padding-box, padding-box,         padding-box;
background-size:       auto auto,   100% 100%,           auto auto;
background-attachment: scroll,      scroll,              scroll;
background-color:      white;

3. Layering Multiple Background Images

The background of a box can have multiple background image layers. The number of layers is determined by the number of comma-separated values in the background-image property. Note that a value of none still creates a layer.

Each of the background images is sized, positioned, and tiled according to the corresponding value in the other background properties. The lists are matched up from the first value: excess values at the end are not used. If a property doesn’t have enough comma-separated values to match the number of layers, the UA must calculate its used value by repeating the list of values until there are enough.

background-image: url(flower.png), url(ball.png), url(grass.png);
background-position: center center, 20% 80%, top left, bottom right;
background-origin: border-box, content-box;
background-repeat: no-repeat;

background-image: url(flower.png), url(ball.png), url(grass.png);
background-position: center center, 20% 80%, top left;
background-origin: border-box, content-box, border-box;
background-repeat: no-repeat, no-repeat, no-repeat;

The first image in the list is the layer closest to the user, the next one is painted behind the first, and so on. The background color, if present, is painted below all of the other layers.

Note: The border-image properties can also define a background image, which, if present, is painted on top of the background layers created by the background properties.

4. Backgrounds of Special Elements

The document canvas is the infinite surface over which the document is rendered. [CSS2] Since no element corresponds to the canvas, in order to allow styling of the canvas CSS propagates the background of the root element (or, in the case of HTML, the <body> element) as described below. However, if the element whose background would be used for the canvas is display: none, then the canvas background is transparent.

If the canvas background is not opaque, the canvas surface below it shows through. The texture of the canvas surface is UA-dependent (but is typically an opaque white).

4.1. The Canvas Background and the Root Element

The background of the root element becomes the canvas background and its background painting area extends to cover the entire canvas. However, any images are sized and positioned relative to the root element’s box as if they were painted for that element alone. (In other words, the background positioning area is determined as for the root element.) The root element does not paint this background again, i.e., the used value of its background is transparent.

4.2. The Canvas Background and the HTML <body> Element

For documents whose root element is an HTML HTML element or an XHTML html element [HTML]: if the computed value of background-image on the root element is none and its background-color is transparent, user agents must instead propagate the computed values of the background properties from that element’s first HTML BODY or XHTML body child element. The used values of that BODY element’s background properties are their initial values, and the propagated values are treated as if they were specified on the root element. It is recommended that authors of HTML documents specify the canvas background using the BODY element rather than the HTML element.

Note: Using containment disables this special handling of the HTML body element. See the CSS Containment 1 § 2 Strong Containment: the contain property for details.

<!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>

4.3. The ::first-line Pseudo-element‘s Background

The ::first-line pseudo-element is like an inline-level element for the purposes of the background (see section 5.12.1 of [CSS2]). That means, e.g., that in a left-justified first line, the background does not necessarily extend all the way to the right edge.

5. Changes

5.1. Additions since [CSS3BG]

• turned background-position into a shorthand and added physical and logical longhands

• added logical keywords to <bg-position>

• added border-area and text values to background-clip

• added background-tbd

• added background-repeat-* longhands

6. Acknowledgments

In addition to the many contributors to the [CSS1], [CSS2], and [CSS3BG] predecessors to this module, the editors would like to thank Tab Atkins, and Håkon Wium Lie for their suggestions and feedback specifically for this Level 4.

Privacy Considerations

No new privacy considerations have been reported on this specification.

Security Considerations

No new security considerations have been reported on this specification.