CSS is a language for describing the rendering of structured documents
(such as HTML and XML) on screen, on paper, in speech, etc. This module
contains the features of CSS level 3 relating to the <image> type
and replaced elements. It includes and extends the functionality of CSS
level 2 [CSS21], which builds on CSS
level 1 [CSS1].
The main extensions compared to level 2 are the generalization of the
<url> type to the <image> type, several additions to the ‘<image>’ type, a
generic sizing algorithm for images and other replaced content in CSS, and
several properties controlling the interaction of replaced elements and
CSS's layout models.
Status of this document
This section describes the status of this document at the time of
its publication. Other documents may supersede this document. A list of
current W3C publications and the latest revision of this technical report
can be found in the W3C technical reports
index at http://www.w3.org/TR/.
Publication as a Working Draft does not imply endorsement by the W3C
Membership. This is a draft document and may be updated, replaced or
obsoleted by other documents at any time. It is inappropriate to cite this
document as other than work in progress.
The (archived) public
mailing list www-style@w3.org (see
instructions) is preferred
for discussion of this specification. When sending e-mail, please put the
text “css3-images” in the subject, preferably like this:
“[css3-images] …summary of
comment…”
In CSS Levels 1 and 2, image values, such as those used in the
‘background-image’ property, could
only be given by a single URI value. This module introduces additional
ways of representing 2D images, for example as a list of URIs denoting
fallbacks, as a reference to an element in the document, or as a gradient.
This module also defines several properties for manipulating raster
images and for sizing or positioning replaced elements such as images
within the box determined by the CSS layout algorithms. It also defines in
a generic way CSS's sizing algorithm for images and other replaced
elements.
1.1. Module Interactions
This module defines and extends the ‘<image>’ value type defined in [[!CSS3VAl]].
Furthermore it replaces the ‘<url>’
type in the ‘background-image’ and
‘list-style-image’ definitions in
CSS1 and CSS2 and adds ‘<image>’ as an alternative to ‘<url>’ in the ‘content’ property's value. It is presumed that
CSS specifications beyond CSS2.1 will use the ‘<image>’ notation in
place of ‘<url>’ where 2D images are
expected. (See e.g. [CSS3BG].)
1.2. Values
This specification follows the CSS property
definition conventions from [CSS21]. Value types not defined in
this specification are defined in CSS Level 2 Revision 1 [CSS21]. Other CSS
modules may expand the definitions of these value types: for example [CSS3COLOR],
when combined with this module, expands the definition of the <color>
value type as used in this specification.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification also accept the inherit
keyword as their property value. For readability it has not been repeated
explicitly.
2. Resolution Units: the
<resolution> type
This specification defines the following units as part of the <resolution> value type:
dpi
dots per inch
dpcm
dots per centimeter
dppx
dots per ‘px’ unit
The <resolution> unit represents the size of a single "dot" in a
graphical representation by indicating how many of these dots fit in a CSS
‘in’, ‘cm’, or ‘px’. For uses, see e.g. the ‘resolution’ media query in [MEDIAQ] or the ‘image-resolution’ property defined below.
Note that due to the 1:96 fixed ratio of CSS ‘in’ to CSS ‘px’,
‘1dppx’ is equivalent to ‘96dpi’. This corresponds to the default resolution
of images displayed in CSS: see ‘image-reslution’.
3. Image Values: the <image> type
The <image> value type denotes a 2D image. It is defined as
...the background of the element will be the portion of the image that
starts at (40px,0px) and is 20px wide and tall, which is just the circle
with a quarter filled in.
* SVG-in-<img> support required. Click the picture to view
the SVG directly.
Note that a legacy UA that doesn't understand the media
fragments notation will ignore the fragment and simply display the
entirety of an image specified with ‘url’. However, since URLs with media fragment
identifiers can also be used in the ‘image()’ notation defined below, authors can take
advantage of CSS's forward-compatible parsing rules to provide a fallback
when using an image fragment URL:
In the example below, the ‘image()’
notation is used together with the media fragment syntax, so that UAs
that don't support media fragments fail to parse the second declaration
and use the first.
background-image: url('swirl.png'); /* old UAs */
background-image: image('sprites.png#xywh=10,30,60,20'); /* new UAs */
3.2. Image Fallbacks and
Annotations: the ‘image()’ function
The ‘image()’ function allows an
author to specify an image with fallback images to be used if the original
image can't be decoded or is a type that the browser doesn't recognize.
Additionally, the author can specify a color as an ultimate fallback to be
used when none of the images can be.
So that authors can take advantage of CSS's forwards-compatible parsing
rules to provide a fallback for image slices, implementations that support
the ‘image()’ notation must
support the xywh=#,#,#,# form of media fragment identifiers
for images. [MEDIA-FRAGS]
Multiple arguments can be given separated by commas, in which case the
function represents the first <string> representing an image that the
browser can successfully load and display. The final argument can specify
a <color> to serve as an ultimate fallback; this can be used, e.g. for
‘background-image’, to ensure
adequate contrast if none of the preceding <image-decl>s can be used.
If the final argument is a <color>, it represents a solid-color image
of the given color with no intrinsic
dimensions.
The rule below would tell the UA to load ‘wavy.svg’ if it can; failing that to load
‘wavy.png’; failing that to display
‘wavy.gif’. For example, the browser
might not understand how to render SVG images, and the PNG may be
temporarily 404 (returning an HTML 404 page, which the browser can't
decode as an image) due to a server move, so the GIF is used until one of
the previous problems corrects itself.
The fallback color can be used to ensure that text is still readable
even when the image fails to load. For example, the following code works
fine if the image is rectangular and has no transparency:
When the image doesn't load, the background color is still there to
ensure that the white text is readable. However, if the image has some
transparency, the black will be visible behind it, which is probably not
desired. The ‘image()’ function
addresses this:
Now, the black won't show at all if the image loads, but if for
whatever reason the image fails, it'll pop in and prevent the white text
from being set against a white background.
At times, one may need a solid-color image for a property or function
that does not accept the <color> type directly. The ‘image()’ function can be used for this: by
specifying only a color without any URLs, the function
immediately falls back to representing a solid-color image of the chosen
color.
In the above, the background is the image "bg-image.png", overlaid with
partially-transparent blue.
Along with each URL, the author may specify a directionality, similar to
adding a dir attribute to an element in HTML. The image
represented by the function takes on the directionality of the used URL.
If a directional image is used on or in an element with opposite
directionality, the image must be flipped in the inline direction (as if
it was transformed by, e.g., scaleX(-1), if the inline
direction is the X axis).
A list may use an arrow for a bullet that points into the content. If
the list can contain both ltr and rtl text, though, the bullet may be on
the left or the right, and an image designed to point into the text on
one side will point out of the text on the other side. This can be fixed
with code like:
<ul style="list-style-image: image("arrow.png" ltr);">
<li dir='ltr'>My bullet is on the left!</li>
<li dir='rtl'>MY BULLET IS ON THE RIGHT!</li>
</ul>
In ltr list items, the image will be used as-is. In the rtl list items,
however, it will be flipped in the inline direction, so it still points
into the content.
3.3. Using Elements as
Images: the ‘element()’ function
The ‘element()’ function allows an
author to use an element in the document as an image. As the referenced
element changes appearance, the image changes as well. This can be used,
for example, to create live previews of the next/previous slide in a
slideshow, or to reference a canvas element for a fancy generated gradient
or even an animated background. The syntax for ‘element()’ is:
where <id-selector> is an ID selector [SELECT], and <identifier> is an
identifer [CSS3VAL].
If the argument to the ‘element()’
function is an ID selector, the function references the element matched by
the selector. If it's an identifier, the function references the element
whose CSS element reference
identifier is the given identifier. (CSS does not define how an
element acquires a CSS
element reference identifier; that is determined by the host
language.)
The ‘element()’ function can be put
to many uses. For example, it can be used to show a preview of the
previous or next slide in a slideshow:
In this example, the navigateSlides function updates
HTML's CSSElementMap to always point to the next and
previous slides, which are then displayed in small floating boxes
alongside the slides. Since you can't interact with the slides through
the ‘element()’ function (it's just an
image), you could even use click handlers on the preview
boxes to help navigate through the page.
Note: A future version of this specification may allow more
than just ID selectors to be passed to ‘element()’, allowing an example like this to be
done with even less javascript - something like ‘background: element(.current-slide + .slide);’.
The image represented by the ‘element()’ function can vary based on a number of
factors. The function must represent the image described by the first set
of conditions, following, that are true:
The function doesn't reference an element
The function represents a solid-color transparent-black image with no
intrinsic dimensions.
The function represents an image with the dimensions and appearance of
the paint source.
The host language defines the dimensions and appearance of paint
sources.
For example, the ‘element()’
function can reference an SVG <pattern> element in an HTML document:
<!DOCTYPE html>
<svg>
<defs>
<pattern id='pattern1'>
<path d='...'>
</pattern>
</defs>
</svg>
<p style="background: element(#pattern1)">
I'm using the pattern as a background!
If the pattern is changed or animated,
my background will be updated too!
</p>
HTML also defines that a handful of elements, such as <canvas>,
<img>, and <video>, provide a paint source. This means that CSS
can, for example, reference a canvas that's being drawn into, but not
included in the page:
<!DOCTYPE html>
<script>
var canvas = document.createElement('canvas');
canvas.width = 20; canvas.height = 20;
document.CSSElementMap.foo = canvas;
drawAnimation(canvas);
</script>
<ul style="list-style-image: element(foo);">
<li>I'm using the canvas as a bullet!</li>
<li>So am I!</li>
<li>As the canvas is changed over time with Javascript,
we'll all update our bullet image with it!</li>
</ul>
The function references an element that is not rendered
The function references an element that is a descendant of a replaced
element
The function represents a solid-color transparent-black image with no
intrinsic dimensions.
For example, all of the following ‘element()’ uses will result in a transparent
background:
<!DOCTYPE html>
<script>
var p = document.createElement('p');
p.textContent = "one";
document.CSSElementMap.one = p;
</script>
<p id='two' style="display:none;">two</p>
<iframe src="http://example.com">
<p id='three'>I'm fallback content!</p>
</iframe>
<ul>
<li style="background: element(one);">
A P element doesn't provide a paint source,
and it's not rendered unless it's in a document.
</li>
<li style="background: element(#two);">
Similarly, a display:none element isn't rendered, and
P still doesn't provide a paint source.
</li>
<li style="background: element(#three);">
The descendants of a replaced element like an IFRAME
can't be used in element() either.
</li>
</ul>
Otherwise
The function represents an image with width and height equal to the
bounding box of the referenced element. The image must be constructed by
rendering the referenced element and its descendants at the same size
that the element would be in its document, over an infinite
transparent-black canvas, positioned so that the edges of the referenced
element's bounding box is flush with the edges of the image. Note: Because images clip anything outside their bounds by
default, this means that decorations that extend outside the bounding
box, like box-shadows or some border-images, may be clipped.
If the referenced element has a transform applied to it or an
ancestor, the transform must be ignored when rendering the element as an
image. If the referenced element is broken across pages, the element
must be displayed as if the page content areas were joined flush in the
pagination direction (elements broken across lines or columns just
render with their bounding box, as normal, which may have unintended
visual effects).
As a somewhat silly example, a <p> element can be reused as a
background elsewhere in the document:
<style>
#src { color: white; background: lime; width: 300px; height: 40px; }
#dst { color: black; background: element(#src); padding: 20px; margin: 20px 0; }
</style>
<p id='src'>I'm an ordinary element!</p>
<p id='dst'>I'm using the previous element as my background!</p>
Implementations may either re-use existing bitmap data generated for the
referenced element or regenerate the display of the element to maximize
quality at the image's size (for example, if the implementation detects
that the referenced element is an SVG fragment); in the latter case, the
layout of the referenced element in the image must not be changed by the
regeneration process. That is, the image must look identical to the
referenced element, modulo rasterization quality.
Host languages may define that some elements provide a paint source. Paint sources have
an intrinsic width, height, and appearance, separate from the process of
rendering, and so may be used as images even when they're not being
rendered. Examples of elements that provide paint sources are the
<linearGradient>, <radialGradient>, and <pattern> elements in
SVG, or the <img>, <video>, and <canvas> elements in HTML.
Detecting and Resolving
Circular Relationships Introduced by ‘element()’
The ‘element()’ function can produce
nonsensical circular relationships, such as an element using itself as its
own background. These relationships can be easily and reliably detected
and resolved, however, by keeping track of a dependency graph and using
common cycle-detection algorithms.
Populate the dependency graph initially by having every element depend
on each of its children. Then, whenever a property on an element A uses
the ‘element()’ function to refer to an
element B, add an edge to the graph by having A depend on B. If a
dependency cycle is detected, any ‘element()’ functions that produced a dependency in
the cycle must represent a fully transparent image with no intrinsic
dimensions.
Someone else needs to review this and make sure that I'm not
missing any cycles.
4. Gradients
A gradient is an image that smoothly fades from one color to another.
These are commonly used for subtle shading in background images, buttons,
and many other things. The two functions described in this section allow
an author to specify such an image in a terse syntax, so that the UA can
generate the image automatically when rendering the page. The syntax of a
<gradient> is:
where <linear-gradient>,
<radial-gradient>, <repeating-linear-gradient>, and
<repeating-radial-gradient> are defined in their applicable
sections below.
Gradients are a type of image, and can be used anywhere an image can,
such as in the ‘background-image’
or ‘list-style-image’ properties.
As with the other <image> types
defined in this specification, gradients can be used in any property that
accepts images. For example:
background: linear-gradient(white, gray);
list-style-image: radial-gradient(circle, #006, #00a 90%,
#0000af 100%, white 100%)
A gradient is drawn into a box with the dimensions of the concrete object size, referred to
as the gradient box.
A gradient has no intrinsic
dimensions. This means that, for example, if you use a gradient in
the ‘background-image’ property
(with ‘background-size’ at the
default value and ‘background-repeat’ not equal to ‘round’), the box will simply be the size of
the background sizing area. Similarly, for a gradient used as a
list-style-image, the box would be a 1em square.
4.1. Linear Gradients
A linear gradient is created by specifying a gradient-line and then
several colors placed along that line. The image is constructed by
creating an infinite canvas and painting it with lines perpendicular to
the gradient-line, with the color of the painted line being the color of
the gradient-line where the two intersect. This produces a smooth fade
from each color to the next, progressing in the specified direction.
The first argument to the function specifies the gradient-line, which gives the gradient a direction
and determines how color-stops are positioned. It may be omitted; if so,
it defaults to "to bottom".
The gradient-line may be specified
in two different ways. The first is by specifying the angle the gradient-line should assume; for the
purposes of this argument, 0deg points upwards, 90deg points toward the
right, and positive angles go clockwise. The starting-point and
ending-point of the gradient-line are
determined by extending a line in both directions from the center of the
gradient box at the angle specified. In
the direction of the angle, the ending-point is the point on the gradient-line where a line drawn
perpendicular to the gradient-line
would intersect the corner of the gradient
box in that direction. The starting-point is determined
identically, except in the opposite direction of the angle.
Alternately, the direction may be specified with keywords that denote
the direction. If the argument is "to top", "to right", "to bottom", or
"to left", the gradient must be rendered identically to "0deg", "90deg",
"180deg", or "270deg", respectively. If the argument specifies a corner to
angle towards, the gradient must be rendered identically to an angle-based
gradient with an angle chosen such that the endpoint of the gradient is in
the same quadrant as the indicated corner, and a line drawn perpendicular
to the gradient-line through the center of the gradient box intersects the two
neighboring corners.
It is expected that the next level of this module will
provide the ability to define the gradient's direction relative to the
current text direction and writing-mode.
This example illustrates visually how to calculate the gradient-line from the rules above.
This shows the starting and ending-point of the gradient-line, along with the actual
gradient, produced by an element with ‘background:
linear-gradient(45deg, white, black);’.
Notice how, though the starting-point and ending-point are outside of
the box, they're positioned precisely right so that the gradient is pure
white exactly at the corner, and pure black exactly at
the opposite corner. That's intentional, and will always be true for
linear gradients.
The gradient's color stops are typically placed between the
starting-point and ending-point on the gradient-line, but this isn't required -
the gradient-line extends infinitely
in both directions. The starting-point and ending-point are merely
arbitrary location markers - the starting-point defines where 0%, 0px, etc
are located when specifying color-stops, and the ending-point defines
where 100% is located. Color-stops are allowed to have positions before 0%
or after 100%.
4.1.2.
Linear Gradient Examples
All of the following ‘linear-gradient()’ examples are presumed to be
backgrounds applied to a box that is 200px wide and 100px tall.
Below are various ways of specifying a basic vertical gradient:
This demonstrates the use of an angle in the gradient. Note that,
though the angle is not exactly the same as the angle between the
corners, the gradient-line is still
sized so as to make the gradient yellow exactly at the upper-left corner,
and blue exactly at the lower-right corner.
This demonstrates a 3-color gradient, and how to specify the location
of a stop explicitly:
linear-gradient(yellow, blue 20%, #0f0);
This demonstrates* a corner-to-corner gradient specified with keywords.
Note how the gradient is red and blue exactly in the bottom-left and
top-right corners, respectively, exactly like the second example.
Additionally, the angle of the gradient is automatically computed so that
the color at 50% (in this case, white) stretches across the top-left and
bottom-right corners.
linear-gradient(to top right, red, white, blue)
* SVG-in-HTML support required to view the image.
4.2. Radial Gradients
In a radial gradient, rather than colors smoothly fading from one side
of the gradient box to the other as
with linear gradients, they instead emerge from a single point and
smoothly spread outward in a circular or elliptical shape.
A radial gradient is specified by indicating the center of the gradient
(where the 0% ellipse will be) and the size and shape of the ending shape (the 100% ellipse). Color stops are
given as a list, just as for ‘linear-gradient()’. Starting from the center
and progressing towards (and potentially beyond) the ending shape concentric ellipses are drawn
and colored according to the specified color stops.
Here is an example of a circular radial gradient 5em wide and
positioned with its center in the top left corner:
radial-gradient(5em circle at top left, yellow, blue)
The arguments are defined as follows:
<position>
Determines the center of the gradient. The <position> notation is defined as the
positioning syntax of a single layer of ‘background-position’
and is resolved in the same way, using the center-point as the subject
and the gradient box as the
positioning area. [CSS21][CSS3BG] If this argument is
omitted, it defaults to ‘center’.
<shape>
Can be either ‘circle’ or
‘ellipse’; determines whether the
gradient's ending shape is a circle or
an ellipse, respectively. If <shape> is
omitted, the ending shape defaults to
a circle if the <size> is a single
<length>, and to an ellipse otherwise.
<size>
Determines the size of the gradient's ending shape. If omitted it defaults to
‘farthest-corner’. It can be given
explicitly or by keyword. For the purpose of the keyword definitions,
consider the gradient box edges as
extending infinitely in both directions, rather than being finite line
segments.
Both ‘circle’ and ‘ellipse’ gradients accept the following keywords
as their <size>:
‘closest-side’
The ending shape is sized so that
that it exactly meets the side of the gradient box closest to the gradient's
center. If the shape is an ellipse, it exactly meets the closest side
in each dimension.
‘farthest-side’
Same as ‘closest-side’, except
the ending shape is sized based on
the farthest side(s).
‘closest-corner’
The ending shape is sized so that
that it passes through the corner of the gradient box closest to the gradient's
center. If the shape is an ellipse, the ending shape is given the same
aspect-ratio it would have if ‘closest-side’ were specified.
‘farthest-corner’
Same as ‘closest-corner’, except
the ending shape is sized based on
the farthest corner. If the shape is an ellipse, the ending shape is given the same aspect
ratio it would have if ‘farthest-side’ were specified.
If <shape> is specified as
‘circle’ or is omitted, the <size> may be given explicitly as:
<length>
Gives the radius of the circle explicitly. Negative values are
invalid.
Note that percentages are not allowed here; they
can only be used to specify the size of an elliptical gradient, not a
circular one. This restriction exists because there is are multiple
reasonable answers as to which dimension the percentage should be
relative to. A future level of this module may provide the ability to
size circles with percentages, perhaps with more explicit controls
over which dimension is used.
If <shape> is specified as
‘ellipse’ or is omitted,
<extent> may instead be given explicitly as:
[<length> | <percentage>]{2}
Gives the size of the ellipse explicitly. The first value represents
the horizontal radius, the second the vertical radius. Percentages
values are relative to the corresponding dimension of the gradient box. Negative values are
invalid.
Expanded with the above definitions, the grammar becomes:
Color-stops are placed on a gradient-ray,
similar to the gradient-line of linear
gradients. The gradient-ray is anchored
at the center of the gradient and extends toward the right. The 0%
location is at the start of the gradient-ray, and the 100% location is on
the point where the gradient-ray
intersects the ending shape. Negative
locations can be specified; though negative locations are never directly
consulted for rendering, they can affect the color of non-negative
locations on the gradient-ray through
interpolation. For example, ‘radial-gradient(red
-50px, yellow 100px)’ produces an elliptical gradient that
starts with a reddish-orange color in the center (specifically, #f50) and
transitions to yellow. Locations greater than 100% simply specify a
location a correspondingly greater distance from the center of the
gradient.
When drawing the concentric ellipses of the gradient, the color of each
ellipse is the color of the gradient-ray at the point where the
ellipse intersects the ray.
4.2.3.
Degenerate Radial Gradients
Some combinations of position, size, and shape will produce a circle or
ellipse with a radius of 0. This will occur, for example, if the center is
on a gradient box edge and ‘closest-side’ or ‘closest-corner’ is specified or if the size and
shape are given explicitly and either of the radiuses is zero. In these
degenerate cases, the gradient must be be rendered as follows:
If the ending shape has zero width
(regardless of the height):
Render as if the ending shape was
an ellipse whose height was an arbitrary very large number and whose
width was an arbitrary very small number greater than zero. This will make the gradient look similar to a horizontal
linear gradient that is mirrored across the center of the ellipse. It
also means that all color-stop positions specified with a percentage
resolve to ‘0px’.
Render as if the ending shape was
an ellipse whose width was an arbitrary very large number and whose
height was an arbitrary very small number greater than zero. This will make the gradient look like a solid-color image
equal to the color of the last color-stop, or equal to the average color
of the gradient if it's repeating.
4.2.4.
Radial Gradient Examples
All of the following examples are applied to a box that is 200px wide
and 100px tall.
These examples demonstrate different ways to write the basic syntax for
radial gradients:
radial-gradient(yellow, green);
radial-gradient(ellipse at center, yellow 0%, green 100%);
radial-gradient(farthest-corner at 50% 50%, yellow, green);
radial-gradient(circle, yellow, green);
radial-gradient(red, yellow, green);
This image shows a gradient originating from somewhere other than the
center of the box:
radial-gradient(farthest-side at left bottom, red, yellow 50px, green);
Here we illustrate a ‘closest-side’ gradient.
radial-gradient(closest-side at 20px 30px, red, yellow, green);
radial-gradient(20px 30px at 20px 30px, red, yellow, green);
radial-gradient(closest-side circle at 20px 30px, red, yellow, green);
radial-gradient(20px 20px at 20px 30px, red, yellow, green);
4.3. Repeating
Gradients
In addition to the ‘linear-gradient()’ and ‘radial-gradient()’ functions, this specification
defines ‘repeating-linear-gradient()’
and ‘repeating-radial-gradient()’
functions. These two functions take the same values and are interpreted
the same as their respective non-repeating siblings defined previously.
When rendered, however, the color-stops are repeated infinitely in both
directions, with their positions shifted by multiples of the difference
between the last specified color-stop's position and the first specified
color-stop's position. For example, ‘repeating-linear-gradient(red 10px, blue 50px)’ is
equivalent to ‘linear-gradient(..., red -30px, blue
10px, red 10px, blue 50px, red 50px, blue 90px, ...)’. Note
that the last color-stop and first color-stop will always coincide at the
boundaries of each group, which will produce sharp transitions if the
gradient does not start and end with the same color.
Repeating gradient syntax is basically identical to that of
non-repeating gradients:
repeating-linear-gradient(red, blue 20px, red 40px)
repeating-radial-gradient(red, blue 20px, red 40px)
repeating-radial-gradient(circle closest-side at 20px 30px, red, yellow, green 100%, yellow 150%, red 200%)
If the distance between the first and last color-stops is non-zero, but
is small enough that the implementation knows that the physical resolution
of the output device is insufficient to faithfully render the gradient,
the implementation must find the average color of the
gradient and render the gradient as a solid-color image equal to
the average color.
If the distance between the first and last color-stops is zero (or
rounds to zero due to implementation limitations), the implementation must
find the average color of a gradient
with the same number and color of color-stops, but with the first and last
color-stop an arbitrary non-zero distance apart, and the remaining
color-stops equally spaced between them. Then it must render the gradient
as a solid-color image equal to that average color.
If the height of a repeating radial gradient is zero, or is close enough
to zero that the implementation knows that the physical resolution of the
output device is insufficient to faithfully render the gradient, the
implementation must find the average color of the
gradient and render the gradient as a solid-color image equal to
the average color.
To find the average color of a gradient,
run these steps:
Define list as an initially-empty list of premultiplied
RGBA colors, and total-length as the distance between first
and last color stops.
For each adjacent pair of color-stops, define weight as
half the distance between the two color-stops, divided by
total-length. Add two entries to list, the first
obtained by representing the color of the first color-stop in
premultiplied sRGBA and scaling all of the components by
weight, and the second obtained in the same way with the
second color-stop.
Sum the entries of list component-wise to produce the
average color, and return it.
As usual, implementations may use whatever algorithm they
wish, so long as it produces the same result as the above.
For example, the following gradient is rendered as a solid light-purple
image (equal to rgb(75%,50%,75%)):
repeating-linear-gradient(red 0px, white 0px, blue 0px);
The following gradient would render the same as the previous under
normal circumstances (because desktop monitors can't faithfully render
color-stops 1/10th of a pixel apart), but would render as a normal
repeating gradient if, for example, the author applied "zoom:100;" to the
element on which the gradient appears:
repeating-linear-gradient(red 0px, white .1px, blue .2px);
Color-stops are points placed along the line defined by the gradient-line at the beginning of the
rule. Color-stops must be specified in order. Percentages refer to the
length of the gradient-line, with 0% being at the starting point and 100%
being at the ending point. Lengths are measured from the starting-point in
the direction of the ending-point. Color-stops are usually placed between
the starting-point and ending-point, but that's not required; the
gradient-line extends infinitely in both directions, and a color-stop can
be placed at any position on the line.
At each color-stop, the line is the color of the color-stop. Between two
color-stops, the line's color is linearly interpolated between the colors
of the two color-stops, with the interpolation taking place in
premultiplied RGBA space. Before the first color-stop, the line is the
color of the first color-stop. After the last color-stop, the line is the
color of the last color-stop.
The following steps must be applied in order to process the
list of color-stops. After applying these rules, all color-stops will have
a definite position and they will be in ascending order:
If the first color-stop does not have a position, set its position to
0%. If the last color-stop does not have a position, set its position to
100%.
If a color-stop has a position that is less than the specified
position of any color-stop before it in the list, set its position to be
equal to the largest specified position of any color-stop before it.
If any color-stop still does not have a position, then, for each run
of adjacent color-stops without positions, set their positions so that
they are evenly spaced between the preceding and following color-stops
with positions.
If multiple color-stops have the same position, they produce an
infinitesimal transition from the one specified first in the rule to the
one specified last. In effect, the color suddenly changes at that position
rather than smoothly transitioning.
Below are several pairs of gradients. The latter of each pair is a
manually "fixed-up" version of the former, obtained by applying the above
rules. For each pair, both gradients will render identically. The numbers in each arrow specify which fixup steps are
invoked in the transformation.
1. linear-gradient(red, white 20%, blue)
=1=>
linear-gradient(red 0%, white 20%, blue 100%)
2. linear-gradient(red 40%, white, black, blue)
=13=>
linear-gradient(red 40%, white 60%, black 80%, blue 100%)
3. linear-gradient(red -50%, white, blue)
=13=>
linear-gradient(red -50%, white 25%, blue 100%)
4. linear-gradient(red -50px, white, blue)
=13=>
linear-gradient(red -50px, white calc(-25px + 50%), blue 100%)
5. linear-gradient(red 20px, white 0px, blue 40px)
=2=>
linear-gradient(red 20px, white 20px, blue 40px)
6. linear-gradient(red, white -50%, black 150%, blue)
=12=>
linear-gradient(red 0%, white 0%, black 150%, blue 150%)
7. linear-gradient(red 80px, white 0px, black, blue 100px)
=23=>
linear-gradient(red 80px, white 80px, black 90px, blue 100px)
The following example illustrates* the difference between a gradient
transitioning in pre-multiplied sRGBA and one transitioning (incorrectly)
in non-premultiplied. In both of these example, the gradient is drawn
over a white background. Both gradients could be written with the
following value:
linear-gradient(90deg, red, transparent, blue)
In premultiplied space, transitions to or from "transparent" always
look nice:
On the other hand, if a gradient were to incorrectly transition in
non-premultiplied space, the colors near "transparent" would noticeably
darken to a grayish color, because "transparent" is actually a shorthand
for ‘rgba(0,0,0,0)’, or transparent
black:
* SVG-in-HTML support required to view the images.
Note: It is recommended that authors not mix different types
of units, such as px, em, or %, in a single rule, as this can cause a
color-stop to unintentionally try to move before an earlier one. For
example, the rule ‘background-image:
linear-gradient(yellow 100px, blue 50%)’ wouldn't require any
fix-up as long as the background area is at least 200px tall. If it was
150px tall, however, the blue color-stop's position would be equivalent to
"75px", which precedes the yellow color-stop, and would be corrected to a
position of 100px.
Note: The definition and implications of "premultiplied"
color spaces are given elsewhere in the technical literature, but a quick
primer is given here to illuminate the process. Given a color expressed as
an rgba() 4-tuple, one can convert this to a premultiplied representation
by multiplying the red, green, and blue components by the alpha component.
For example, a partially-transparent blue may be given as
rgba(0,0,255,.5), which would then be expressed as [0, 0, 127.5, .5] in
its premultiplied representation. Interpolating colors using the
premultiplied representations rather than the plain rgba representations
tends to produce more attractive transitions, particularly when
transitioning from a fully opaque color to fully transparent. Note that
transitions where either the transparency or the color are held constant
(for example, transitioning between rgba(255,0,0,100%) and
rgba(0,0,255,100%) or rgba(255,0,0,100%) and rgba(255,0,0,0%)) have
identical results whether the color interpolation is done in premultiplied
or non-premultiplied color-space. Differences only arise when both the
color and transparency differ between the two endpoints.
5. Sizing Images and Objects in CSS
Images used in CSS may come from a number of sources, from binary image
formats (such as gif, jpeg, etc), dedicated markup formats (such as SVG),
and CSS-specific formats (such as the linear-gradient() value type defined
in this specification). As well, a document may contain many other types
of objects, such as video, plugins, or nested documents. These images and
objects (just objects hereafter) may offer many
types of sizing information to CSS, or none at all. This section defines
generically the size negotiation model between the object and the CSS
layout algorithms.
5.1. Object-Sizing
Terminology
In order to define this handling, we define a few terms, to make it
easier to refer to various concepts:
intrinsic dimensions
An object's intrinsic dimensions are its preferred, natural width,
height, and aspect ratio, if they exist. There can be an intrinsic height and intrinsic width, defining a definite rectangle
and an intrinsic aspect
ratio. (Most bitmap images fall into this category.) There can
be only an intrinsic aspect ratio
defining the relation of the width to the height, but no definite size.
(SVG images designed to scale may fall into this category.) There can be
just an intrinsic height or width. Or there can be no intrinsic
dimensions at all, implying that the object has no preferred size or
aspect ratio. (Embedded documents are often assumed to have no intrinsic
size, as are CSS gradients, defined in this specification.)
If an object (such as an icon) has multiple sizes, then the largest
size is taken as its intrinsic size. If it has multiple aspect ratios at
that size, or has multiple aspect ratios and no size, then the aspect
ratio closest to the aspect ratio of the default object size is used.
specified size
The specified size of an object is given by CSS, such as through the
‘width’ and ‘height’ or ‘background-size’ properties. The specified
size can be a definite width and height, a set of constraints, or a
combination thereof.
The following list defines the default object size of properties
and contexts that appear in CSS 2.1, plus ‘border-image’ from CSS 3 Backgrounds &
Borders. Newer modules that accept an ‘<image>’ component value in a new context
must define the default object
size in that context.
When an image or object is specified in a document, such as through a
‘url()’ value in a ‘background-image’ property or an
src attribute on an <img> element, CSS
queries the object for its intrinsic
dimensions.
If the specified size has only a
width or height, but not both, then the concrete object size is given
that specified width or height. The other dimension is calculated as
follows:
If the object has an intrinsic
aspect ratio, the missing dimension of the concrete object size is
calculated using the intrinsic aspect-ratio and the present
dimension.
If the specified size has neither
a definite width nor height, and has no additional contraints, the
dimensions of the concrete object
size are calculated as follows:
If the specified size has
additional constraints, the concrete
object size must be sized to satisfy those constraints. For
example, the ‘min-width’,
‘min-height’, ‘max-width’, and ‘max-height’ properties specify slightly more
complex handling for sizing replaced elements, and ‘background-repeat: round’ can further adjust the
size calculated by ‘background-size’ so that the image fits a
whole number of times into the background positioning area.
The ‘object-fit’ property specifies how the
contents of a replaced element should be scaled relative to the box
established by its used height and width. It also enables scaling a
replaced element up to a specified maximum size or down to a specified
minimum size while preserving its aspect ratio.
Not all replaced elements can be scaled, but images typically can.
If the replaced element's content does not have an intrinsic aspect ratio, all of
the values for ‘object-fit’ are treated as ‘fill’. Otherwise, the object is scaled as follows:
This will make the contents exactly fill the replaced element's
content box.
contain
Determine the used ‘height’
and ‘width’ of the element as
usual, except: If both ‘height’
and ‘width’ are ‘auto’, and the used value of at least one of
‘max-width’ and ‘max-height’ is not ‘none’, then compute the element's used width and
used height as though the intrinsic dimensions of the contents were
infinitely large numbers whose ratio is the actual intrinsic ratio of
the contents. This will proportionally scale the used width and height
up to the given maximum constraints.
Set the concrete object
size to the largest width and height that has the same aspect
ratio as the object's intrinsic aspect ratio, and additionally has
neither width nor height larger than the replaced element's used width
and height, respectively.
cover
Determine the used ‘height’
and ‘width’ of the element as
usual, except: If both ‘height’
and ‘width’ are ‘auto’, and the used value of at least one of
‘min-width’ and ‘min-height’ is not ‘none’, then compute the element's used width and
used height as though the intrinsic dimensions of the contents were
infinitely small numbers whose ratio is the actual intrinsic ratio of
the contents. This will proportionally scale the used width and height
down to the given minimum constraints.
Set the concrete object
size to the smallest width and height that has the same aspect
ratio as the object's intrinsic aspect ratio, and additionally has
neither width nor height smaller than the replaced element's used width
and height, respectively.
Size the content as if ‘none’ or
‘contain’ were specified, whichever
would result in a smaller concrete
object size.
Note that both ‘none’
and ‘contain’ respect the content's
intrinsic aspect ratio, so the concept of "smaller" is well-defined.
If the content does not completely fill the replaced element's content
box, the unfilled space shows the replaced element's background. Since
replaced elements always clip their contents to the content box, the
content will never overflow. See the ‘object-position’ property for positioning
the object with respect to the content box.
An example showing how four of the values of ‘object-fit’
cause the replaced element (blue figure) to be scaled to fit its
height/width box (shownwith a green background), using the initial value
for ‘object-position’. The fifth value,
‘scale-down, in this case
looks identical to ’contain.
Note: the ‘object-fit’ property has similar semantics
to the fit attribute in [SMIL10].
Note: Per the CSS?Object
Negotiation algorithm, the concrete
object size (or, in this case, the size of the content) does not
directly scale the object itself - it is merely passed to the object as
information about the size of the visible canvas. How to then draw into
that size is up to the image format. In particular, raster images always
scale to the given size, while SVG uses the given size as the size of the
"SVG Viewport" (a term defined by SVG) and then uses the values of several
attributes on the root <svg> element to determine how to draw itself.
User agents MAY accept ‘image-fit’ as an alias for ‘object-fit’, as
a previous version of this specification used that name. Authors must not
use ‘image-fit’ in their
stylesheets.
The ‘object-position’ property determines the
alignment of the replaced element inside its box. The <position> value type is defined as
the positioning syntax of ‘background-position’
and is resolved in the same way, using the image as the subject and the
content box as the positioning area. [CSS21][CSS3BG]
Note that areas of the box not covered by the replaced
element will show the element's background.
User agents MAY accept ‘image-position’ as an alias for ‘object-position’, as a previous version of
this specification used that name. Authors must not use ‘image-position’ in their stylesheets.
6. Image Processing
6.1. Overriding Image
Resolutions: the ‘image-resolution’ property
The image resolution is defined as the number of image pixels per
unit length, e.g., pixels per inch. Some image formats can record
information about the resolution of images. This information can be
helpful when determining the actual size of the image in the formatting
process. However, the information can also be wrong, in which case it
should be ignored. By default, CSS assumes a resolution of one image pixel
per CSS ‘px’ unit; however, the
‘image-resolution’ property allows using
some other resolution.
Name:
image-resolution
Value:
[from-image || <resolution>] && snap?
Initial:
1dppx
Applies to:
all elements
Inherited:
yes
Media:
visual
Computed value:
as specified, except with <resolution> possibly altered by
computing for ‘snap’ (see below)
The ‘image-resolution’ property specifies the
intrinsic resolution of all raster images used in or on the element. (As
vector formats such as SVG do not have an intrinsic resolution, this
property has no effect on vector images.) The intrinsic resolution of an image is used to
determine the image's intrinsic size. Affected images include
images in the element's content (e.g. replaced elements and/or generated
content), background images,list markers, etc. Values have the following
meanings:
<resolution>
Specifies the intrinsic resolution explicitly.
from-image
The image's intrinsic resolution is taken as that specified by the
image format. If the image does not specify its own resolution, the
explicitly specified resolution is used (if given), else it defaults to
‘1ddpx’.
snap
If the "snap" keyword is provided, the computed ‘<resolution>’
(if any) is the specified resolution rounded to the nearest value that
would map one image pixel to an integer number of device pixels. If the
resolution is taken from the image, then the used intrinsic resolution is
the image's native resolution similarly adjusted.
Printers tend to have substantially higher resolution than computer
monitors; due to this, an image that looks fine on the screen may look
pixellated when printed out. The ‘image-resolution’ property can be used to
embed a high-resolution image into the document and maintain an
appropriate size, ensuring attractive display both on screen and on
paper:
img.high-res {
image-resolution: 300dpi;
}
With this set, an image meant to be 5 inches wide at 300dpi will
actually display as 5in wide; without this set, the image would display
as approximately 15.6in wide since the image is 15000 image pixels
across, and by default CSS displays 96 image pixels per inch.
Some image formats can encode the image resolution into the image data.
This rule specifies that the UA should use the image resolution found in
the image itself, falling back to 1 image pixel per CSS ‘px’ unit.
img { image-resolution: from-image }
These rules both specify that the UA should use the image resolution
found in the image itself, but if the image has no resolution, the
resolution is set to 300dpi instead of the default ‘1dppx’.
Using this rule, the image resolution is set to 300dpi. (The resolution
in the image, if any, is ignored.)
img { image-resolution: 300dpi }
This rule, on the other hand, if used when the screen's resolution is
96dpi, would instead render the image at 288dpi (so that 3 image pixels
map to 1 device pixel):
img { image-resolution: 300dpi snap; }
The ‘snap’ keyword can also be used
when the resolution is taken from the image. In this rule:
img { image-resolution: snap from-image; }
An image declaring itself as 300dpi will, in the situation above,
display at 288dpi (3 image pixels per device pixel) whereas an image
declaring 72dpi will render at 96dpi (1 image pixel per device pixel).
6.2. Orienting an Image
on the Page: the ‘image-orientation’ property
Sometimes images from camera phones, digital cameras or scanners are
encoded sideways. For example, the first row of image data can represent
the leftmost or rightmost column of image pixels. Furthermore, often such
devices have limited resources, and do not have the capability to rotate
the image into an upright orientation. However, this type of device may
have internal knowledge or can accept input from its user as to the
rotational correction to perform. The ‘image-orientation’ property provides a way
to apply an ”out-of-band“ rotation to image source data to
correctly orient an image.
Note this facility is not intended to specify layout
transformations such as arbitrary rotation or flipping the image in the
horizontal or vertical direction. (See [CSS3-2D-TRANSFORMS] for
a feature designed to do that.) It is also not needed to correctly orient
an image when printing in landscape versus portrait orientation, as that
rotation is done as part of layout. (See [CSS3PAGE].) It should only be used
to correct incorrectly-oriented images.
Name:
image-orientation
Value:
<angle>
Initial:
0deg
Applies to:
images
Inherited:
no
Media:
visual
Computed value:
specified value, rounded and normalized (see text)
This property specifies an orthogonal rotation to be applied to an image
before it is laid out. CSS layout processing applies to the image
after rotation. This implies, for example:
The intrinsic height and width are derived from the rotated rather
than the original image dimensions.
The height (width) property applies to the vertical (horizontal)
dimension of the image, after rotation.
Positive values cause the image to be rotated to the right (in a
clockwise direction), while negative values cause a rotation to the left.
The computed value of the property is calculated by rounding the specified
angle to the nearest quarter-turn (90deg, 100grad, .25turn, etc.),
rounding away from 0 (that is, 45deg is rounded to 90deg, while -45deg is
rounded to -90deg), then moduloing the value by 1 turn (360deg, 400grad,
etc.).
If the image itself is transformed in some way (for example, if the
content of an element is provided by the ‘image()’ function with a directionality opposite
the element's directionality), the image's transformation must be applied
before ‘image-orientation’ is. As well, ‘image-orientation’ must be applied before
any further transformation of the element, such as through CSS Transforms.
Note that in CSS, orientation data encoded in the image (e.g.
EXIF data) is ignored. Is this an issue? What do
printers do?
The following example rotates the image 90 degrees clockwise:
The same effect could be achieved with, for example, an angle of
-270deg or 450deg.
7. Conformance
7.1. 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.
7.2. Conformance
Classes
Conformance to CSS Image Values and Replaced Content Module Level 3 is
defined for three conformance classes:
A style sheet is conformant to CSS Image Values and Replaced Content
Module Level 3 if all of its declarations that use properties defined in
this module have values that are valid according to the generic CSS
grammar and the individual grammars of each property as given in this
module.
A renderer is conformant to CSS Image Values and Replaced Content Module
Level 3 if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined by CSS
Image Values and Replaced Content Module Level 3 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 CSS Image Values and Replaced Content
Module Level 3 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.
7.3. 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.
7.4. Experimental
Implementations
To avoid clashes with future CSS features, the CSS2.1 specification
reserves a prefixed
syntax for proprietary and experimental extensions to CSS.
Prior to a specification reaching the Candidate Recommendation stage in
the W3C process, all implementations of a CSS feature are considered
experimental. The CSS Working Group recommends that implementations use a
vendor-prefixed syntax for such features, including those in W3C Working
Drafts. This avoids incompatibilities with future changes in the draft.
7.5. Non-Experimental
Implementations
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.
Acknowledgments
Thanks to the Webkit team, Brad Kemper, Brian Manthos, and Alan Gresley
for their contributions to the definition of gradients; to Melinda Grant
for her work on ‘object-fit’, ‘object-position’, and ‘image-orientation’; to Robert O'Callahan
for the definition of ‘element()’; and
to Michael Day, Håkon Lie, and Shinyu Murakami for ‘image-resolution’.
![[9 circles, with 0 to 8 eighths filled in]](sprites.svg){kind=icon}
![[An image showing a box with a background shading gradually from white in the bottom-left corner to black in the top-right corner. There is a line, illustrating the gradient-line, angled at 45 degrees and passing through the center of the box. The starting-point and ending-point of the gradient-line are indicated by the intersection of the gradient-line with two additional lines that pass through the bottom-left and top-right corners of the box.]](gradient-diagram.png)
