This draft contains the features of CSS relating to borders and backgrounds. The main extensions compared to level 2 are borders consisting of images, boxes with multiple backgrounds, boxes with rounded corners and boxes with shadows.
CSS is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, etc.
Status of this document
This section describes the status of this document at the time of its publication.
A list of current W3C publications
and the latest revision of this technical report
can be found in the W3C technical reports index at https://www.w3.org/TR/.
This document was published
by the CSS Working Group as a Candidate Recommendation Draft using the Recommendation
track.
Publication as a Candidate Recommendation
does not imply endorsement by W3C and its Members.
A Candidate Recommendation Draft
integrates changes from the previous Candidate Recommendation
that the Working Group intends to include in a subsequent Candidate Recommendation Snapshot.
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.
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including the spec code "css-backgrounds" in the title, like this:
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1. Introduction
This subsection is not normative.
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 decoration of the border area and with the background of the content, padding, and border areas.
Additionally the box may be given a "drop-shadow" effect
with the box-shadow property.
If an element is broken into multiple box fragments, box-decoration-break[CSS-BREAK-3] 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 module replaces and extends the background and border features
defined in [CSS2] sections 8.5 and 14.2.
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. 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. 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.
For example, this set of declarations:
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;
has exactly the same effect as this set,
with the extra position dropped
and the missing values for background-origin and background-repeat filled in (emphasized for clarity):
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.
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
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.
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.
Some examples specifying background images:
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).
This property specifies how background images are tiled
after they have been sized and positioned.
The property’s value is given as a comma-separated list
of <repeat-style> values where
The image is repeated as often as will fit
within the background positioning area without being clipped,
and then the 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 axis,
in which case only one image is placed,
and background-position determines its position in this axis.
round
The image is repeated 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 this direction.
Unless one of the two keywords is 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: repeat-y;
background-position: center;
}
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
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.
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 § 2.11 Backgrounds of Special Elements for the cases when
background images are not clipped.)
Thus, unless the image is tiled, it may be invisible.
This example creates an infinite vertical band
that remains "glued" to the viewport when the document is scrolled.
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;
}
If only one value is specified,
the second value is assumed to be center.
If two values are given,
a <length-percentage> as the first value represents
the horizontal position (or offset)
and a <length-percentage> as the second value represents
the vertical position (or offset).
The <length-percentage> values here represent
an offset of the top left corner of the background image
from the top left corner of the background positioning area.
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.
The following declarations give the stated (horizontal, vertical)
offsets from the top left corner:
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 */
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.
[画像:Diagram of image position within element]
Diagram of the meaning of background-position: 75% 50%.
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.
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.
The following background shorthand declarations use keywords
to set background-position to the stated percentage values.
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% */
In the example below, the (single) image is placed
in the lower-right corner of the viewport.
Background positions can also be relative to other corners than the top left.
For example, the following puts the background image
10px from the bottom and 3em from the right:
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.
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.)
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.
Here are some examples.
The first example stretches the background image
independently in both dimensions
to completely cover the content area:
The second example stretches the image
so that exactly two copies fit horizontally.
The aspect ratio is preserved:
p {
background-image: url(tubes.png);
background-size: 50% auto;
background-origin: border-box }
This example forces the background image to be 15 by 15 pixels:
p {
background-size: 15px 15px;
background-image: url(tile.png)}
This example uses the image’s natural size.
Note that this is the only possible behavior in CSS level 1 and 2.
body {
background-size: auto; /* default */
background-image: url(flower.png) }
The following example rounds the height of the image to 33.3%,
up from the specified value of 30%.
At 30%, three images would fit entirely and a fourth only partially.
After rounding, three images fit exactly.
The width of the image is 20% of the background positioning area width
and is not rounded.
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.
In this example the background image is shown at its natural size:
div {
background-image: url(image1.png);
background-repeat: repeat; /* default */
background-size: auto } /* default */
In the following example, the background is shown with a width of
3em and its height is scaled proportionally to keep the original
aspect ratio:
In the following example,
the background is shown with a width of approximately 3em:
scaled so that it fits a whole number of times in the width of the background.
The height is scaled proportionally to keep the original aspect ratio:
div {
background-image: url(image3.png);
background-repeat: round repeat;
background-size: 3em auto }
In the following example, the background image is shown
with a width of 3em and a height that is either
the height corresponding to that width at the original aspect ratio
or slightly less:
div {
background-image: url(image4.png);
background-repeat: repeat round;
background-size: 3em auto }
In the following example,
the background image is shown with a height of approximately 4em:
scaled slightly so that it fits a whole number of times in the background height.
The width is the approximately the width that corresponds to
a 4em height at the original aspect ratio:
scaled slightly so that it fits a whole number of times
in the background width.
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.)
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.
In the first rule of the following example, only a value for background-color has been given and the
other individual properties are set to their initial values. In the
second rule, many individual properties have been specified.
body { background: red }
p { background: url("chess.png") 40% / 10em gray
round fixed border-box; }
The following example shows how a both
a background color (#CCC) and a background image (url(metal.jpg))
are set.
The image is rescaled to the full width of the element:
E { background: #CCC url("metal.jpg") top left / 100% auto no-repeat}
Another example shows equivalence:
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 }
The following declaration with multiple, comma-separated values
background: url(a.png) top left no-repeat,
url(b.png) center / 100% 100% no-repeat,
url(c.png) white;
is equivalent to
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;
2.11. 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).
2.11.1. 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.
2.11.2. 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.
According to these rules, the canvas underlying the following HTML document
will have a "marble" background:
<!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>
2.11.3. 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.
3. Borders
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, various properties define
the style (border-style),
color (border-color),
and thickness (border-width)
of the border.
If there is only one component value,
it applies to all sides.
If there are two values,
the top and bottom are set to the first value
and the right and left are set to the second.
If there are three values,
the top is set to the first value,
the left and right are set to the second,
and the bottom is set to the third.
If there are four values
they apply to the top, right, bottom, and left, respectively.
If there is only one component value,
it applies to all sides.
If there are two values,
the top and bottom are set to the first value
and the right and left are set to the second.
If there are three values,
the top is set to the first value,
the left and right are set to the second,
and the bottom is set to the third.
If there are four values
they apply to the top, right, bottom, and left, respectively.
The style is specified as a <line-style> keyword, where
No border.
Color and width are ignored (i.e., the border has width 0).
Note this means that the initial value of border-image-width will also resolve to zero.
hidden
Same as none,
but has different behavior in the border conflict resolution rules
for border-collapsed tables [CSS2].
dotted
A series of round dots.
dashed
A series of square-ended 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.)
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 specified border-color.)
Note: Border colors close to black or white
may need different color calculations than colors in between
in order to create the required "3D" effect
of groove, ridge, inset, or outset.
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.
If there is only one component value,
it applies to all sides.
If there are two values,
the top and bottom are set to the first value
and the right and left are set to the second.
If there are three values,
the top is set to the first value,
the left and right are set to the second,
and the bottom is set to the third.
If there are four values
they apply to the top, right, bottom, and left, respectively.
Note: Although the initial width is medium,
the initial style is none;
therefore the used initial width is 0.
The border property is a shorthand property for setting
the same border-width, border-color, and border-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.
The border shorthand also resets border-image to its initial value.
It is therefore recommended that authors use the border shorthand,
rather than other shorthands or the individual properties,
to override any border settings earlier in the cascade.
This will ensure that border-image has also been reset
to allow the new styles to take effect.
Note: The CSS Working Group intends for the border shorthand
to reset all border properties in future levels of CSS as well.
For example, if a border-characters property
is introduced in the future to allow glyphs as borders,
it will also be reset by the border shorthand.
By using the border shorthand to reset borders,
authors can be guaranteed a "blank canvas"
no matter what properties are introduced in the future.
For example, the first rule below is equivalent to the set of five
rules shown after it:
p { border: solid red }
p {
border-top: solid red;
border-right: solid red;
border-bottom: solid red;
border-left: solid red;
border-image: none;
}
Since, to some extent, the properties have overlapping functionality,
the order in which the rules are specified is important.
Consider this example:
blockquote {
border-color: red;
border-left: double;
color: black
}
In the above example,
the color of the left border is black, while the other borders are red.
This is due to border-left setting the width, style, and color.
Since the color value is not given by the border-left property,
it will be taken from the color property.
The fact that the color property is set after the border-left property
is not relevant.
The two <length-percentage> values of the border-*-radius properties
define the radii of
a quarter ellipse that defines the shape of the corner
of the outer border edge (see the diagram below).
The first value is the horizontal radius,
the second the vertical radius.
If the second value is omitted it is copied from the first.
If either length is zero, the corner is square, not rounded.
Percentages for the horizontal radius refer to the width of the border box,
whereas percentages for the vertical radius refer to the height of the border box.
Negative values are invalid.
The border-radius shorthand sets all four border-*-radius properties.
If values are given before and after the slash,
then the values before the slash set the horizontal radii
and the values after the slash set the vertical radii.
If there is no slash, then the values set the radii in both axes equally.
The four values for each radii are given in the order
top-left, top-right, bottom-right, bottom-left.
If bottom-left is omitted it is the same as top-right.
If bottom-right is omitted it is the same as top-left.
If top-right is omitted it is the same as top-left.
The padding edge (inner border) radius is
the outer border radius minus the corresponding border thickness.
In the case where this results in a negative value,
the inner radius is zero.
(In such cases the center of the border’s inner curve might not coincide
with that of its outer curve.)
Likewise the content edge radius is
the padding edge radius minus the corresponding padding,
or if that is negative, zero.
The border and padding thicknesses in the curved region
are thus interpolated from the adjoining sides,
and when two adjoining borders are of different thicknesses
the corner will show a smooth transition
between the thicker and thinner borders.
All border styles (solid, dotted, inset, etc.)
follow the curve of the border.
[画像:The effect of rounded corners on unequal borders]
The effect of a rounded corner when the two borders it connects
are of unequal thickness (left)
and the effect of a rounded corner on borders
that are thicker than the radius of the corner (right).
Note: If the center of a corner’s outer curve
is past an opposite padding edge (in the border area of a side opposite the corner),
the inner curve will not be a full quarter ellipse.
The margin edge, being outside the border edge,
calculates its radius by adding the corresponding margin thickness
to each border radius.
However, in order to create a sharper corner when the border radius is small
(and thus ensure continuity between round and sharp corners),
when the border radius is less than the margin,
the margin is multiplied by the proportion 1 + (r-1)3,
where r is the ratio of the border radius to the margin,
in calculating the corner radii of the margin box shape.
4.3. Corner Clipping
Although border images are not affected by border-radius,
other effects that clip painting or event handling
to the border, padding, or content edge
must clip to their respective curves.
For example,
backgrounds clip to the curve specified by background-clip, overflow values other than visible to the curved padding edge (when overflow on both axes is not visible), replaced element content to the curved content edge,
pointer events to the curved border edge,
etc.
Note: As border-radius reduces the interactive area of an element
authors should make sure the remaining interactive area conforms
to recommended minima for the platforms they target;
in particular, conforming to recommended minimum touch target sizes
may require larger widths and heights when border-radius is used.
This example adds appropriate padding,
so that the contents do not overflow the corners.
Note that there is no border,
but the background will still have rounded corners.
Color and style transitions must be contained
within the segment of the border that intersects
the smallest rectangle that contains both border radii
as well as the center of the inner curve
(which may be a point representing the corner of the padding edge,
if the border radii are smaller than the border width).
If one of these borders is zero-width,
then the other border takes up the entire transitional area.
Otherwise,
the center of color and style transitions between adjoining borders
is a point along the curve that is a continuous monotonic function
of the ratio of the border widths.
However it is not defined what these transitions look like
or what function maps from this ratio to a point on the curve.
[画像:Illustration of the transition region on curved corners]
Given these corner shapes, color and style transitions
must be contained within the green region.
In case D the rectangle defined by the border radii
does not include the center of the inner curve (which is a sharp corner),
so the transition region is expanded to include that corner.
Transitions may take up the entire transition region,
but are not required to:
For example, a gradient color transition between two solid border styles
might take up only the region bounded by
the tips of the outer radii and the tips of the inner radii
(represented in case D by the dark green region).
4.5. Overlapping Curves
Corner curves must not overlap:
When the sum of any two adjacent border radii
exceeds the size of the border box,
UAs must proportionally reduce
the used values of all border radii
until none of them overlap.
The algorithm for reducing radii is as follows:
Let f = min(Li/Si),
where i ∈ {top, right, bottom, left}, Si is the sum of the two corresponding radii
of the corners on side i,
and Ltop = Lbottom = the width of the box,
and Lleft = Lright = the height of the box.
If f < 1, then all corner radii are reduced
by multiplying them by f.
Note: This formula ensures that quarter circles remain quarter circles
and large radii remain larger than smaller ones,
but it may reduce corners that were already small enough,
which may make borders of nearby elements that should look the same
look different.
If the curve interferes with UI elements such as scrollbars,
the UA may further reduce the used value of the affected border radii
(and only the affected border radii)
as much as necessary, but no more.
For example, the borders A of the figure below might be the result of
all corners need to be reduced by a factor 0.8 to make them fit.
The used border radii thus are
0.4em (instead of 0.5em) and 1.6em (instead of 2em).
See borders B in the figure.
Authors can specify an image to be used in place of the border styles.
In this case, the border’s design is taken from the sides and corners
of an image specified with border-image-source,
whose pieces may be sliced, scaled, and stretched in various ways
to fit the size of the border image area.
The border-image properties do not affect layout:
layout of the box, its content, and surrounding content
is based on the border-width and border-style properties only.
This example creates
a top and bottom border consisting of a whole number of orange diamonds
and a left and right border of a single, stretched diamond.
The corners are diamonds of a different color.
The image to tile is as follows.
Apart from the diamonds, it is transparent:
Here, even though the border-width is 12px,
the border-image-width property computes to 124px.
The border image area is then outset 31px from the border box and into the margin area.
If the border-image fails to load
(or if border images are not supported by the UA),
the fallback rendering uses a green double border.
Notice that the border shorthand resets border-image.
This makes it easy to turn off or reset all border effects:
.notebox {
border: double orange;
/* must set 'border' shorthand first, otherwise it erases 'border-image' */
border-image: url("border.png") 30 round;
/* but other 'border' properties can be set after */
border-width: thin thick;
}
...
.sidebar .notebox {
box-shadow: 0 0 5px gray;
border-radius: 5px;
border: none; /* turn off all borders */
/* 'border' shorthand resets 'border-image' */
}
Specifies an image to use as a border
in place of the rendering specified by the border-style properties
and, if given the fill keyword in border-image-slice,
as an additional image backdrop for the element.
If the value is none or if the image cannot be displayed
(or the property doesn’t apply), the border styles will be used;
otherwise the element’s border-style borders are not drawn
and this border image is drawn
as described in the sections below.
This property specifies inward offsets from
the top, right, bottom, and left edges of the image,
dividing it into nine regions: four corners, four edges and a middle.
The middle image part is discarded (treated as fully transparent)
unless the fill keyword is present.
(It is drawn over the background;
see Drawing the Border Image.)
If there is only one component value,
it applies to all sides.
If there are two values,
the top and bottom are set to the first value
and the right and left are set to the second.
If there are three values,
the top is set to the first value,
the left and right are set to the second,
and the bottom is set to the third.
If there are four values
they apply to the top, right, bottom, and left, respectively.
Numbers represent pixels in the image
(if the image is a raster image)
or vector coordinates
(if the image is a vector image).
fill
The fill keyword, if present,
causes the middle part of the border-image to be preserved.
(By default it is discarded, i.e., treated as empty.)
Negative values are invalid.
Computed values larger than the size of the image are interpreted as 100%.
The regions given by the border-image-slice values may overlap.
However 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 nonempty transparent image
had been specified for those parts.
Analogously for the top and bottom values.
The border image is drawn inside an area called the border image area.
This is an area whose boundaries by default correspond to the border box,
see border-image-outset.
The four values of border-image-width specify offsets
that are used to divide the border image area into nine regions.
The offsets represent inward distances from
the top, right, bottom, and left sides of the area, respectively.
If there is only one component value,
it applies to all sides.
If there are two values,
the top and bottom are set to the first value
and the right and left are set to the second.
If there are three values,
the top is set to the first value,
the left and right are set to the second,
and the bottom is set to the third.
If there are four values
they apply to the top, right, bottom, and left, respectively.
If two opposite border-image-width offsets are large enough that they overlap,
then the used values of all border-image-width offsets
are proportionally reduced until they no longer overlap.
In mathematical notation:
Given Lwidth as the width of the border image area, Lheight as its height,
and Wside as the border-image-width offset for the side side,
let f = min(Lwidth/(Wleft+Wright), Lheight/(Wtop+Wbottom)).
If f < 1,
then all W are reduced by multiplying them by f.
If there is only one component value,
it applies to all sides.
If there are two values,
the top and bottom are set to the first value
and the right and left are set to the second.
If there are three values,
the top is set to the first value,
the left and right are set to the second,
and the bottom is set to the third.
If there are four values
they apply to the top, right, bottom, and left, respectively.
Represents an outset of the specified multiple of
the corresponding computed border-width.
Negative values are invalid.
Portions of the border-image that are rendered outside the border box do not trigger scrolling.
Also such portions are invisible to mouse events
and do not capture such events on behalf of the element.
Note: Even though they never cause a scrolling mechanism,
outset images may still be clipped by an ancestor or by the viewport.
This property specifies how the images for the sides
and the middle part of the border image are scaled and tiled.
The first keyword applies to the horizontal scaling and tiling
of the top, middle and bottom parts,
the second to the vertical scaling and tiling
of the left, middle and right parts;
see Drawing the Border Image.
If the second keyword is absent, it is assumed to be the same as the first.
Values have the following meanings:
stretch
The image is stretched to fill its corresponding region.
repeat
The image is tiled (repeated) to fill its corresponding region.
round
The image is tiled (repeated) to fill its corresponding region.
If it does not fill the area with a whole number of tiles,
the image is rescaled so that it does.
space
The image is tiled (repeated) to fill its corresponding region.
If it does not fill the region with a whole number of tiles,
the extra space is distributed around the tiles.
The exact process for scaling and tiling the border-image parts
is given in the section below.
The two images for the top and bottom edges are made as tall
as the top and bottom border image regions, respectively,
and their width is scaled proportionally.
The images for the left and right edges are made as wide
as the left and right border image regions, respectively,
and their height is scaled proportionally.
The corner images are scaled to be as wide and as tall
as the their respective border image regions.
The middle image’s width is scaled by the same factor as the top image
unless that factor is zero or infinity,
in which case the scaling factor of the bottom is substituted,
and failing that, the width is not scaled.
The height of the middle image is scaled by the same factor as the left image
unless that factor is zero or infinity,
in which case the scaling factor of the right image is substituted,
and failing that, the height is not scaled.
If the first keyword is stretch,
the top, middle and bottom images are further scaled
to be as wide as the middle region of the border image area.
The height is not changed any further.
If the first keyword is round,
the top, middle and bottom images are resized in width,
so that exactly a whole number of them fit
in the middle region of the border image area,
exactly as for round in the background-repeat property.
If the first keyword is repeat or space,
the top, middle, and bottom images are not changed any further.
The effects of stretch, round, repeat, and space for the second keyword are analogous,
acting on the height of the left, middle and right images.
Position the first tile.
If the first keyword is repeat,
the top, middle, and bottom images
are centered horizontally in their respective regions.
Otherwise the images are placed at the left edge
of their respective regions of the border image area.
If the second keyword is repeat,
the left, middle, and right images
are centered vertically in their respective regions.
Otherwise the images are placed at the top edge
of their respective regions of the border image area.
Tile and draw.
The images are then tiled to fill their respective regions.
In the case of space,
any partial tiles are discarded and the extra space distributed
before, after, and between the tiles.
(I.e. the gap before the first tile,
the gap after the last tile,
and the gaps between tiles are equalized.) This can result in empty border-image side regions.
The images are drawn at the same stacking level as normal borders:
immediately in front of the background layers.
The border-image properties apply
to the border of tables and inline tables
that have border-collapse set to collapse.
However, this specification does not define
how such an image border is rendered.
In particular, it does not define how the image border interacts
with the borders of cells, rows and row groups at the edges of the table
(see border conflict resolution in [CSS2]).
It is expected that a future specification will define the rendering.
It is recommended that UAs do not apply border images
to tables with collapsed borders until then.
by computed value,
treating none as a zero-item list
and appending blank shadows (transparent 0 0 0 0)
with a corresponding inset keyword as needed
to match the longer list
if the shorter list is otherwise compatible with the longer one
The box-shadow property attaches one or more drop shadows to the box.
The property accepts either
the none value,
which indicates no shadows,
or a comma-separated list of shadows, ordered front to back.
Each shadow is given as a <shadow>,
represented by 2-4 length values, an optional color, and an optional inset keyword.
Omitted lengths are 0; omitted colors default to currentColor.
Specifies the blur radius.
Negative values are invalid.
If the blur value is zero, the shadow’s edge is sharp.
Otherwise, the larger the value, the more the shadow’s edge is blurred.
See Shadow Blurring, below.
Specifies the spread distance.
Positive values cause the shadow to expand in all directions by the specified radius.
Negative values cause the shadow to contract.
See Shadow Shape, below.
Note that for inner shadows,
expanding the shadow (creating more shadow area)
means contracting the shadow’s perimeter shape.
Specifies the color of the shadow.
If the color is absent, it defaults to currentColor.
inset
If present, the inset keyword changes the drop shadow
from an outer box-shadow (one that shadows the box onto the canvas,
as if it were lifted above the canvas)
to an inner box-shadow (one that shadows the canvas onto the box,
as if the box were cut out of the canvas and shifted behind it).
The example below demonstrates the effects of spread and blur on the shadow:
An outer box-shadow casts a shadow
as if the border-box of the element were opaque.
Assuming a spread distance of zero, its perimeter has
the exact same size and shape as the border box.
The shadow is drawn outside the border edge only:
it is clipped inside the border-box of the element.
An inner box-shadow casts a shadow
as if everything outside the padding edge were opaque.
Assuming a spread distance of zero, its perimeter has
the exact same size and shape as the padding box.
The shadow is drawn inside the padding edge only:
it is clipped outside the padding box of the element.
If a spread distance is defined,
the shadow perimeter defined above
is expanded outward (for outer box-shadows)
or contracted inward (for inner box-shadows)
by outsetting (insetting, for inner shadows)
the shadow’s straight edges by the spread distance (and flooring the resulting width/height at zero).
Below are some examples of an orange box with a blue border
being given a drop shadow.
To preserve the box’s shape when spread is applied,
the corner radii of the shadow are also increased (decreased, for inner shadows)
from the border-box (padding-box) radii by adding (subtracting)
the spread distance (and flooring at zero).
However, in order to create a sharper corner when the border radius is small
(and thus ensure continuity between round and sharp corners),
when the border radius is less than the spread distance (or in the case of an inner shadow,
less than the absolute value of a negative spread distance),
the spread distance is first multiplied by the proportion 1 + (r-1)3,
where r is the ratio of the border radius to the spread distance,
in calculating the corner radii of the spread shadow shape.
For example, if the border radius is 10px and the spread distance is 20px (r = .5),
the corner radius of the shadow shape will be 10px + 20px ×ばつ (1 + (.5 - 1)3) = 27.5px
rather than 30px.
This adjustment is applied independently to the radii in each dimension.
The border-image does not affect the shape of the box-shadow.
6.1.2. Blurring Shadow Edges
A non-zero blur radius indicates
that the resulting shadow should be blurred,
such as by a Gaussian filter.
The exact algorithm is not defined;
however the resulting shadow must approximate
(with each pixel being within 5% of its expected value)
the image that would be generated by applying to the shadow
a Gaussian blur with a standard deviation equal to half the blur radius.
Note: This means for a long, straight shadow edge,
the blur radius will create a visibly apparent color transition
approximately the twice length of the blur radius
that is perpendicular to and centered on the shadow’s edge,
and that ranges
from almost the full shadow color at the endpoint inside the shadow
to almost fully transparent at the endpoint outside it.
6.1.3. Layering, Layout, and Other Details
The shadow effects are applied front-to-back:
the first shadow is on top and the others are layered behind.
Shadows do not influence layout and may overlap (or be overlapped by)
other boxes and text or their shadows.
In terms of stacking contexts and the painting order,
the outer box-shadows of an element are drawn immediately below the background of that element,
and the inner shadows of an element are drawn immediately above the background of that element
(below the borders and border image, if any).
Unless otherwise specified, drop shadows are only applied to the principal box.
If the affected box has multiple fragments,
the shadows are applied as specified in box-decoration-break.
Outer shadows have no effect on internal table elements in the collapsing border model.
If a shadow is defined for single border edge in the collapsing border model
that has multiple border thicknesses
(e.g. an outer shadow on a table where one row has thicker borders than the others,
or an inner shadow on a rowspanning table cell that adjoins cells with different border thicknesses),
the exact position and rendering of its shadows are undefined
7. Levels
This section is informative. CSS has different levels of features, each a subset of the other.
(See [CSS-2017] for a full explanation.)
The lists below describe which features from this specification are in each level.
Fixed the definition for where box shadows apply.
(Issue 9286)
8.2. Changes since the 22 December 2020 Candidate Recommendation Snapshot
Clarified that the rule about not propagating backgrounds from the root
when it doesn’t generate boxes only applies to display: none,
not display: contents.
(Issue 3779)
Spread radius adjustment is only applied to shadows and margins
where the radius of curvature grows, rather than shrinks.
(Issue 1900)
To preserve the box’s shape when spread is applied,
the corner radii of the shadow are also increased (decreased, for inner shadows)
from the border-box radii by adding (subtracting)
the spread distance (and flooring at zero).
However, in order to create a sharper corner when the border radius is small
(追記) (and thus ensure continuity between round and sharp corners) (追記ここまで)
,
when the border radius is less than the spread distance
(追記) (or in the case of an inner shadow,
less than the absolute value of a negative spread distance) (追記ここまで)
,
the spread distance is multiplied by the proportion 1 + (r-1)3,
where r is the ratio of the border radius to the spread distance,
in calculating the corner radii of the spread shadow shape.
Clarified that an omitted <color> in a <shadow> defaults to currentColor,
not some mysterious unnamed value with the same behavior.
(2766)
If the color is absent,
(追記) it defaults to currentColor (追記ここまで)
(削除) the used color is taken from the color property. (削除ここまで)
Cleaned up and regularized "Animation type" and "Computed value" lines
in the property definition tables.
Changed syntax to use the new CSS bracketed range notation to reflect the prose restrictions on negative values,
and corrected a few grammar definition errors introduced in the previous publication.
Minor editorial improvements.
8.4. Changes since the 9 September 2014 Candidate Recommendation
Added missing definition of box-shadow: none.
(Apparently this was obvious enough that nobody noticed it was missing until now.)
Moved <'background-color'> component of <final-bg-layer> to the front for serialization
because some authors seem to expect this even though it makes less sense?
Dropped effect of border-radius from collapsed-borders tables.
Renamed <position> back to <bg-position> since other properties will be eliding the three-value syntax.
8.5. Changes since the 4 February 2014 Last Call Working Draft
Allow <color> and inset to be interleaved in any order in box-shadow,
since they are not ambiguous and CSS generally allows variant ordering where not ambiguous.
Define gradually increasing corner radius formula for box-shadow spread curvature
to create continuity between sharp corners (border-radius = 0)
and curved corners (border-radius> spread distance).
This also gives better results for all intermediate states.
Add definition for how the margin edge is curved in response to border-radius.
(This is relevant for [CSS-SHAPES], but does not change conformance to CSS Backgrounds and Borders Level 3.)
Tighten up the definition of spread for box-shadow.
Clarify that a border-style of none also implies an initial border-image-width of zero
(since border-image-width is initially set to the computed border-width, which in this case is zero).
Section 3.4: Fixed the
incorrect definition of the computed value of background-repeat. The value is always a pair of keywords, never a
single keyword.
If both values are auto then the natural width and/or
height of the image should be used, if any
(追記) , the missing
dimension (if any) behaving as ‘auto’ as described above (追記ここまで)
.
(削除) The center of color and style transitions between
adjoining borders is at the point on the curve that is at an angle
that is proportional to the ratio of the border widths. For
example, if the top and right border widths are equal, that point
is at a 45° angle from the horizontal, and if the top is twice the
width of the right the point is at a 30° angle from the
horizontal. The line demarcating this transition is drawn between
the point at that angle on the outer arc and the point at that
angle on the inner arc. (削除ここまで)
(追記) If one of these borders is zero-width, then the other
border takes up the entire transitional area. Otherwise, the
center of color and style transitions between adjoining borders
must be proportional to the ratio of the border widths such that a
function of its location is continuous with respect to this ratio.
However it is not defined what these transitions look like or how
"proportional" maps to a point on the curve. (追記ここまで)
(追記) If the image must be sized to determine the slices (for
example, for SVG images with no natural size), then it is sized
as for an auto-sized background, using the border image area as
the default object size in place of the background positioning
area. (追記ここまで)
(追記) UAs may also apply ‘box-decoration-break’ to control
rendering at bidi-imposed breaks, i.e. when bidi reordering causes
an inline to split into non-contiguous fragments. Otherwise such
breaks are always handled as ‘slice’. (追記ここまで)
(削除) If one or two values are specified, for a <length> the
absolute length, otherwise a percentage. If three or four values
are specified, two pairs of a keyword plus a length or
percentage. (削除ここまで)
(追記) Two keywords representing the origin and
two offsets from that origin, each given as an absolute length (if
given a <length>), otherwise as a percentage. (追記ここまで)
Changed
(削除) Equivalent (削除ここまで)
to
(追記) Computes (追記ここまで)
in
definition of keywords.
Added "Animation Type" line to property definition tables.
Section 3.6:
Renamed <bg-position> production to <position> for
easier re-use in other specifications and recast the grammar to be more
verbose but easier to understand.
Section 6.5:
Added space keyword to border-image-repeat property value table:
it was included in the list of allowable values, but not in the syntax
definition.
Removal of recommendation to use gradients for color transitions
when border-radius produces a curve.
(Re)Addition of box-shadow property.
Various clarifications.
9. Acknowledgments
Tapas Roy was editor of the Border Module,
before it was merged with the Background Module.
Thanks to Ben Stucki for defining what happens with rounded corners
if the two adjoining borders are of unequal thickness or one of them is zero;
to Arjan Eising and Anne van Kesteren for the border-radius syntax;
to Zack Weinberg for the corner transition regions diagram;
and to Lea Verou, plinss, and dbaron for the corner radius adjustment formula
(with special thanks to Lea for the live demo).
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.)
Finally, special thanks go to Brad Kemper for his feedback and suggestions
for many of the features in the draft,
for drawing all the box-shadow examples,
and for proposing some radical changes to the border-image property
that solved a number of problems with the earlier definition.
10. Privacy Considerations
This specification introduces no new privacy considerations.
11. Security Considerations
This specification introduces no new security considerations.
Conformance
Document conventions
Conformance requirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words "MUST",
"MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this
document are to be interpreted as described in RFC 2119.
However, for readability, these words do not appear in all uppercase
letters in this specification.
All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. [RFC2119]
Examples in this specification are introduced with the words "for example"
or are set apart from the normative text with class="example",
like this:
This is an example of an informative example.
Informative notes begin with the word "Note" and are set apart from the
normative text with class="note", like this:
Note, this is an informative note.
Advisements are normative sections styled to evoke special attention and are
set apart from other normative text with <strong class="advisement">, like
this: UAs MUST provide an accessible alternative.
Conformance classes
Conformance to this specification
is defined for three conformance classes:
A style sheet is conformant to this specification
if all of its statements that use syntax defined in this module are valid
according to the generic CSS grammar and the individual grammars of each
feature defined in this module.
A renderer is conformant to this specification
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by this specification by parsing them correctly
and rendering the document accordingly. However, the inability of a
UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.)
An authoring tool is conformant to this specification
if it writes style sheets that are syntactically correct according to the
generic CSS grammar and the individual grammars of each feature in
this module, and meet all other conformance requirements of style sheets
as described in this module.
Partial implementations
So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers must treat as invalid (and ignore
as appropriate) any at-rules, properties, property values, keywords,
and other syntactic constructs for which they have no usable level of
support. In particular, user agents must not selectively
ignore unsupported component values and honor supported values in a single
multi-value property declaration: if any value is considered invalid
(as unsupported values must be), CSS requires that the entire declaration
be ignored.
Implementations of Unstable and Proprietary Features
Once a specification reaches the Candidate Recommendation stage,
non-experimental implementations are possible, and implementors should
release an unprefixed implementation of any CR-level feature they
can demonstrate to be correctly implemented according to spec.
To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental
CSS renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before
releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS
Working Group.
For this specification to be advanced to Proposed Recommendation,
there must be at least two independent, interoperable implementations
of each feature. Each feature may be implemented by a different set of
products, there is no requirement that all features be implemented by
a single product. For the purposes of this criterion, we define the
following terms:
independent
each implementation must be developed by a
different party and cannot share, reuse, or derive from code
used by another qualifying implementation. Sections of code that
have no bearing on the implementation of this specification are
exempt from this requirement.
interoperable
passing the respective test case(s) in the
official 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 user
agent (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:
implements the specification.
is available to the general public. The implementation may
be a shipping product or other publicly available version
(i.e., beta version, preview release, or "nightly build").
Non-shipping product releases must have implemented the
feature(s) for a period of at least one month in order to
demonstrate stability.
is not experimental (i.e., a version specifically designed
to pass the test suite and is not intended for normal usage
going forward).
The specification will remain Candidate Recommendation for at least
six months.
by computed value,
treating none as a zero-item list
and appending blank shadows (transparent 0 0 0 0)
with a corresponding inset keyword as needed
to match the longer list
if the shorter list is otherwise compatible with the longer one
per grammar
either the keyword none or a list,
each item consisting of four absolute lengths
plus a computed color and optionally also a inset keyword
![画像:[image: rectangle with two tiny rounded corners and two very large ones, on opposite corners]](/index.cgi/speech/https://www.w3.org/TR/css-backgrounds-3/images/corner-large-mix.png){kind=link}
