CSS Images Module Level 4

Editor’s Draft,

Specification Metadata
This version:
https://drafts.csswg.org/css-images-4/
Latest published version:
https://www.w3.org/TR/css4-images/
Previous Versions:
Issue Tracking:
Tracker
Inline In Spec
GitHub Issues
Editors:
Tab Atkins Jr. (Google)
Elika J. Etemad / fantasai (Invited Expert)
Lea Verou (Invited Expert)

Abstract

This module contains the features of CSS level 4 relating to the <image> type and replaced elements. It includes and extends the functionality of CSS level 2 [CSS21] and in the previous level of this specification [css3-images]. The main extensions compared to "CSS Images Module Level 3" [css3-images] are several additions to the <image> type, such as the image() notation, the element() notation, and conic gradients.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.

Status of this document

This is a public copy of the editors’ draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

GitHub Issues are preferred for discussion of this specification. When filing an issue, please put the text “css-images” in the title, preferably like this: “[css-images] …summary of comment…”. All issues and comments are archived, and there is also a historical archive.

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

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 March 2017 W3C Process Document.

1. Introduction

This section is not normative.

This module introduces additional ways of representing 2D images, for example as a URL with color fallback, as conic gradients, or as the rendering of another element in the document.

2. Image Values: the <image> type

The <image> value type denotes a 2D image. It can be a url reference, image notation, or gradient notation. Its syntax is:

<image> = <url> | <image()> | <image-set()> | <cross-fade()> | <gradient>

An <image> can be used in many CSS properties, including the background-image, list-style-image, cursor properties [CSS21] (where it replaces the <url> component in the property’s value).

In some cases, an image is invalid, such as a <url> pointing to a resource that is not a valid image format. An invalid image is rendered as a solid-color transparent image with no intrinsic dimensions. However, invalid images have special behavior in some contexts, such as the image() notation.

2.1. Image File Formats

At minimum, the UA must support the following image file formats when referenced from an <image> value, for all the properties in which using <image> is valid:

The UA may support other file formats as well.

2.2. Image References: the url() notation

Note: No change from [css3-images].

2.3. Resolution Negotiation: the image-set() notation

Delivering the most appropriate image resolution for a user’s device can be a difficult task. Ideally, images should be in the same resolution as the device they’re being viewed in, which can vary between users. However, other factors can factor into the decision of which image to send; for example, if the user is on a slow mobile connection, they may prefer to receive lower-res images rather than waiting for a large proper-res image to load. The image-set() function allows an author to ignore most of these issues, simply providing multiple resolutions of an image and letting the UA decide which is most appropriate in a given situation.

This solution assumes that resolution is a proxy for filesize, and therefore doesn’t appropriately handle multi-resolution sets of vector images, or mixing vector images with raster ones (e.g. for icons). For example, use a vector for high-res, pixel-optimized bitmap for low-res, and same vector again for low-bandwidth (because it’s much smaller, even though it’s higher resolution).

The syntax for image-set() is:

image-set() = image-set( <image-set-option># )
<image-set-option> = [ <image> | <string> ] <resolution>

We should add "w" and "h" dimensions as a possibility, and a "format()" function, to match the functionality of HTML’s picture.

The image-set() function can not be nested inside of itself, either directly or indirectly (as an argument to another <image> type).

Is this restriction needed?

Each <string> inside image-set() represents a <url>.

Every <image-set-option> in a given image-set() must have a different <resolution>, or else the function is invalid.

UAs must make a UA-specific choice of which <image-set-option> to load, based on whatever criteria they find relevant (such as the resolution of the display, connection speed, etc). The image-set() then represents the image associated with the URL of that choice. The image’s intrinsic resolution is the resolution associated with that choice. UAs may change which <image-set-option> they wish to use for a given image-set() over the lifetime of the page, if the criteria used to determine which option to choose change significantly enough to make it worthwhile in the UA’s estimation.

This example shows how to use image-set() to provide an image in three versions: a "normal" version, a "high-res" version, and an extra-high resolution version for use in high-quality printing (as printers can have extremely high resolution):
background-image: image-set( "foo.png" 1x,
                             "foo-2x.png" 2x,
                             "foo-print.png" 600dpi );

2.4. Image Fallbacks and Annotations: the image() notation

The image() function allows an author to:

The image() notation is defined as:

image() = image( <image-tags>? [ <image-src>? , <color>? ]! )
<image-tags> = [ ltr | rtl ]
<image-src> = [ <url> | <string> ]

A <string> used in image() represents a <url>. As usual for URLs in CSS, relative URLs are resolved to an absolute URL (as described in Values & Units [css-values-3]) when a specified image() value is computed.

If the image has an orientation specified in its metadata, such as EXIF, the UA must rotate or flip the image to correctly orient it as the metadata specifies.

2.4.1. Image Fallbacks

If both a URL and a <color> are specified in image(), then whenever the URL represents an invalid image, the image() function renders as if the URL were not specified at all; it generates a solid-color image as specified in §2.4.3 Solid-color Images.

The fallback color can be used to ensure that text is still readable even when the image fails to load. For example, the following legacy code works fine if the image is rectangular and has no transparency:
body      { color: black; background: white; }
p.special { color: white; background: url("dark.png") black; }

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:

body      { color: black; background: white; }
p.special { color: white; background: image("dark.png", black); }

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.

2.4.2. Image Fragments

When a URL specified in image() represents a portion of a resource (e.g. by the use of media fragment identifiers) that portion is clipped out of its context and used as a standalone image.

For example, given the following image and CSS:

[9 circles, with 0 to 8 eighths filled in]

background-image: image('sprites.svg#xywh=40,0,20,20')

...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.

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 specified via image(). [MEDIA-FRAGS]

Note that image fragments can also be used with the url() notation. However, a legacy UA that doesn’t understand the media fragments notation will ignore the fragment and simply display the entirety of the image.

Since the image() notation requires UAs to support media fragments, authors can take advantage of CSS’s forward-compatible parsing rules to provide a fallback when using an image fragment URL:

background-image: url('swirl.png'); /* old UAs */
background-image: image('sprites.png#xywh=10,30,60,20'); /* new UAs */

If a URL uses a fragment identifier syntax that the implementation does not understand, or does not consider valid for that type of image, the URL must be treated as representing an invalid image.

Note: This error-handling is limited to image(), and not in the definition of URL, for legacy compat reasons.

2.4.3. Solid-color Images

If the image() function is specified with only a <color> argument (no URL), it represents a solid-color image of the specified color with no intrinsic dimensions.

For example, one can use this as a simple way to "tint" a background image, by overlaying a partially-transparent color over the top of the other image:
background-image: image(rgba(0,0,255,.5)), url("bg-image.png");

background-color does not work for this, as the solid color it generates always lies beneath all the background images.

2.4.4. Bidi-sensitive Images

Before listing any <image-src>s, the author may specify a directionality for the image, similar to adding a dir attribute to an element in HTML. If a directional image is used on or in an element with opposite direction, 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).

Note: Absent this declaration, images default to no directionality at all, and thus don’t care about the directionality of the surrounding element.

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(ltr 'arrow.png');">
  <li dir='ltr'>My bullet is on the left!</li>
  <li dir='rtl'>MY BULLET IS ON THE RIGHT!</li>
</ul>

This should render something like:

⇒ My bullet is on the left!
  !THGIR EHT NO SI TELLUB YM ⇐

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.

2.5. Resolution Negotiation: the image-set() notation

Note: No change from [css3-images].

2.6. Combining images: the cross-fade() notation

Note: No change from [css3-images].

2.7. Using Elements as Images: the element() notation

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.

Note: The element() function only reproduces the appearance of the referenced element, not the actual content and its structure. Authors should only use this for decorative purposes, and must not use element() to reproduce an element with significant content across the page. Instead, just insert multiple copies of the element into the document.

The syntax for element() is:

element() = element( <id-selector> )

where <id-selector> is an ID selector [SELECT].

Do we need to be able to refer to elements in external documents (such as SVG paint servers)? Or is it enough to just use url() for this?

This name conflicts with a somewhat similar function in GCPM. This needs to be resolved somehow.

Want the ability to do "reflections" of an element, either as a background-image on the element or in a pseudo-element. This needs to be specially-handled to avoid triggering the cycle-detection.

When we have overflow:paged, how can we address a single page in the view?

The element() function references the element matched by its argument. The ID is first looked up in the elementSources map, as described in that section. If it’s not found, it’s then matched against the document. If multiple elements are matched, the function references the first such element.

The image represented by the element() function can vary based on whether the element is visible in the document:

an element that is rendered, is not a descendant of a replaced element, and generates a stacking context
The function represents an image with its intrinsic size equal to the decorated bounding box of the referenced element:

Note: Because images clip anything outside their bounds by default, this means that decorations that extend outside the decorated bounding box, like box shadows, may be clipped.

The image is constructed by rendering the referenced element and its descendants (at the same size that they would be in the document) over an infinite transparent canvas, positioned so that the edges of the decorated bounding box are flush with the edges of the image.

Requiring some degree of stacking context on the element appears to be required for an efficient implementation. Do we need a full stacking context, or just a pseudo-stacking context? Should it need to be a stacking context normally, or can we just render it as a stacking context when rendering it to element()?

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. [CSS3-TRANSFORMS]

If the referenced element is broken across pages, the element is displayed as if the page content areas were joined flush in the pagination direction, with pages' edges corresponding to the initial containing block’s start edge aligned. Elements broken across lines or columns are just rendered with their decorated bounding box.

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.

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; position: relative; }
#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>

an element that is not rendered, but which provides a paint source
The function represents an image with the intrinsic size and appearance of the paint source. The host language defines the size 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 displayed in the page:

<!DOCTYPE html>
<script>
  var canvas = document.querySelector('#animated-bullet');
  canvas.width = 20; canvas.height = 20;
  drawAnimation(canvas);
</script>
<canvas id='animated-bullet' style='display:none'></canvas>
<ul style="list-style-image: element(#animated-bullet);">
  <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>
anything else

The function represents an invalid image.

For example, all of the following element() uses will result in a transparent background:

<!DOCTYPE html>
<p id='one' style="display:none; position: relative;">one</p>
<iframe src="http://example.com">
  <p id='two' style="position: relative;">I’m fallback content!</p>
</iframe>
<ul>
  <li style="background: element(#one);">
    A display:none element isn’t rendered, and a P element
    doesn’t provide a paint source.
  </li>
  <li style="background: element(#two);">
    The descendants of a replaced element like an IFRAME
    can’t be used in element() either.
  </li>
  <li style="background: element(#three);">
    There’s no element with an id of "three", so this also
    gets rendered as a transparent image.
  </li>
</ul>

An element is not rendered if it does not have an associated box. This can happen, for example, if the element or an ancestor is display:none. Host languages may define additional ways in which an element can be considered not rendered; for example, in SVG, any descendant of a <defs> element is considered to be not rendered.

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:

<!DOCTYPE html>
<script>
function navigateSlides() {
  var currentSlide = ...;
  document.querySelector('#prev-slide').id = '';
  document.querySelector('#next-slide').id = '';
  currentSlide.previousElementSibling.id = 'prev-slide';
  currentSlide.nextElementSibling.id = 'next-slide';
}
</script>
<style>
.slide {
  /* Need to be a stacking context to be element()-able. */
  position: relative;
}
#prev-preview, #next-preview {
  position: fixed;
  ...
}
#prev-preview { background: element(#prev-slide); }
#next-preview { background: element(#next-slide); }
</style>
<a id='prev-preview'>Previous Slide</a>
<a id='next-preview'>Next Slide</a>
<section class='slide'>...</section>
<section class='slide current-slide'>...</section>
...

In this example, the navigateSlides function updates the ids of 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.

2.7.1. Paint Sources

Host languages may define that some elements provide a paint source. Paint sources have an intrinsic appearance and can obtain a concrete object size without having to do layout or rendering, and so may be used as images even when they’re not rendered.

In HTML, the <img>, <video>, and <canvas> elements provide paint sources (defined in each element’s section in HTML5).

In SVG, any element that provides a paint server provides a paint source. Note: In SVG1.1, the <linearGradient>, <radialGradient>, and <pattern> elements provide paint sources. They are drawn as described in the spec, with the coordinate systems defined as follows:

objectBoundingBox
The coordinate system has its origin at the top left corner of the rectangle defined by the concrete object size that it’s being drawn into, and the same width and height as the concrete object size. A single user coordinate is the width and height of the concrete object size.
userSpaceOnUse
The coordinate system has its origin at the top left corner of the rectangle defined by the concrete object size that it’s being drawn into, and the same width and height as the concrete object size. User coordinates are sized equivalently to the CSS px unit.

Note: It is expected that a future version of this module will define ways to refer to paint sources in external documents, or ones that are created solely by script and never inserted into a document at all.

2.7.2. Using Out-Of-Document Sources: the ElementSources interface

The element() function normally selects elements within a document, but elements that provide a paint source don’t necessarily need to be in-document. For example, an HTML <canvas> element can be created, maintained, and drawn into entirely in script, with no need for it to be inserted into the document directly.

All that’s needed is a way to refer to the element, as an ID selector cannot select elements outside of the document. The elementSources Map object provides this.

partial interface CSS {
  [SameObject] readonly attribute Map elementSources;
};

Any entries in the elementSources map with a string key and a value that is an object providing a paint source are made available to the element() function.

Whenever element() uses an <id-selector>, the ID’s value (without the leading # character) is first looked up in the elementSources map:

This reuse of the ID selector matches Moz behavior. I’m trying to avoid slapping a <custom-ident> right in the beginning of the grammar, as that eats too much syntax-space. Another possibility, though, is to start the value with a language-defined keyword followed by a <custom-ident>, like element(external fancy) or something. Naming suggestions welcome.

For example, fancy animating backgrounds can be done with an external canvas:
<script>
var bg = document.createElement('canvas');
bg.height = 200;
bg.width = 1000;
drawFancyBackground(bg);
CSS.elementSources.set('fancy', bg);
</script>
<style>
h1 {
  background-image: element(#fancy);
}
</style>

As the "fancy" canvas is drawn into and animated, the backgrounds of all the H1 elements will automatically update in tandem.

Note that the elementSources map is consulted before the document to match the ID selector, so even if there’s an element in the document that would match #fancy, the backgrounds will still predictably come from the elementSources value instead.

2.7.3. Cycle Detection

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.

The dependency graph consists of edges such that:

If the graph contains a cycle, any element() functions participating in the cycle are invalid images.

3. 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 gradient notations 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:

<gradient> = [
  <linear-gradient()> | <repeating-linear-gradient()> |
  <radial-gradient()> | <repeating-radial-gradient()> |
  <conic-gradient()>  | <repeating-conic-gradient()> ]

As with the other <image> types defined in this specification, gradients can be used in any property that accepts images. For example:

A gradient is drawn into a box with the dimensions of the concrete object size, referred to as the gradient box. However, the gradient itself has no intrinsic dimensions.

For example, if you use a gradient as a background, by default the gradient will draw into a gradient box the size of the element’s padding box. If background-size is explicitly set to a value such as 100px 200px, then the gradient box will be 100px wide and 200px tall. Similarly, for a gradient used as a list-style-image, the box would be a 1em square, which is the default object size for that property.

Gradients are specified by defining the starting point and ending point of a gradient line (which, depending on the type of gradient, may be technically a line, or a ray, or a spiral), and then specifying colors at points along this line. The colors are smoothly blended to fill in the rest of the line, and then each type of gradient defines how to use the color of the gradient line to produce the actual gradient.

3.1. Linear Gradients: the linear-gradient() notation

Note: No change from [css3-images].

3.2. Radial Gradients: the radial-gradient() notation

Note: No change from [css3-images].

3.3. Conic Gradients: the conic-gradient() notation

A conic gradient starts by specifying the center of a circle, similar to radial gradients, except that conic gradient color-stops are placed around the circumference of the circle, rather than on a line emerging from the center, causing the color to smoothly transition as you spin around the center, rather than as you progress outward from the center.

A conic gradient is specified by indicating a rotation angle, the center of the gradient, and then specifying a list of color-stops. Unlike linear and radial gradients, whose color-stops are placed by specifying a <length>, the color-stops of a conic gradient are specified with an <angle>. Rays are then drawn emerging from the center and pointing in all directions, with the color of each ray equal to the color of the gradient-line where they intersect it.

Note: These gradients are called "conic" or "conical" because, if the color stops are chosen to be significantly lighter on one side than the other, it produces a pattern that looks like a cone observed from above. They are also known as "angle" gradients in some contexts, since they are produced by varying the rotation angle of a ray.

[An image showing a box with a background shading gradually clockwise from white to black, starting from the top. A gradient circle is shown, and the colors at 0 and 216 degrees respectively.]

This example visually illustrates how conic-gradient(at 25% 30%, white, black 60%) would be drawn. Note that since color stop positions always resolve to angles, the only effect of the center center is a 2D translation of the gradient, i.e. it does not affect how the gradient is drawn.

3.3.1. conic-gradient() Syntax

The syntax for a conic gradient is:

conic-gradient() = conic-gradient(
  [ from <angle> ]? [ at <position> ]?,
  <angular-color-stop-list>
)

The arguments are defined as follows:

<angle>
The entire gradient is rotated by this angle. If omitted, defaults to 0deg. The unit identifier may be omitted if the <angle> is zero.
<position>
Determines the gradient center of the gradient. The <position> value type (which is also used for background-position) is defined in [css-values-3], and is resolved using the center-point as the object area and the gradient box as the positioning area. If this argument is omitted, it defaults to center.

Usually in conic gradients the sharp transition at 0deg is undesirable, which is typically avoided by making sure the first and last color stops are the same color. Perhaps it would be useful to have a keyword for automatically achieving this.

Would a radius (inner & outer) for clipping the gradient be useful? If so, we could also support lengths in color stop positions, since we now have a specific radius.

Are elliptical conic gradients useful? Do graphics libraries support them?

3.3.2. Placing Color Stops

Color stops are placed on a gradient line that curves around the center in a circle, with both the 0% and 100% locations at 0deg. Just like linear gradients, 0deg points to the top of the page, and increasing angles correspond to clockwise movement around the circle.

Note: It may be more helpful to think of the gradient line as forming a spiral, where only the segment from 0deg to 360deg is rendered. This avoids any confusion about "overlap" when you have angles outside of the rendered region.

A color-stop can be placed at a location before 0% or after 100%; though these regions are never directly consulted for rendering, color stops placed there can affect the color of color-stops within the rendered region through interpolation or repetition (see repeating gradients). For example, conic-gradient(red -50%, yellow 150%) produces a conic gradient that starts with a reddish-orange color at 0deg (specifically, #f50), and transitions to an orangish-yellow color at 360deg (specifically, #fa0).

The color of the gradient at any point is determined by first finding the unique ray anchored at the center of the gradient that passes through the given point. The point’s color is then the color of the gradient line at the location where this ray intersects it.

3.3.3. Conic Gradient Examples

All of the following conic-gradient() examples are presumed to be applied to a box that is 300px wide and 200px tall, unless otherwise specified.

Below are various ways of specifying the same basic conic gradient:
background: conic-gradient(#f06, gold);
background: conic-gradient(at 50% 50%, #f06, gold);
background: conic-gradient(from 0deg, #f06, gold);
background: conic-gradient(from 0deg at center, #f06, gold);
background: conic-gradient(#f06 0%, gold 100%);
background: conic-gradient(#f06 0deg, gold 1turn);

Below are various ways of specifying the same basic conic gradient. This demonstrates how even though color stops with angles outside [0deg, 360deg) are not directly painted, they can still affect the color of the painted part of the gradient.
background: conic-gradient(white -50%, black 150%);
background: conic-gradient(white -180deg, black 540deg);
background: conic-gradient(hsl(0,0%,75%), hsl(0,0%,25%));

Below are two different ways of specifying the same rotated conic gradient, one with a rotation angle and one without:
background: conic-gradient(from 45deg, white, black, white);
background: conic-gradient(hsl(0,0%,87.5%), white 45deg, black 225deg, hsl(0,0%,87.5%));

Note that offsetting every color stop by the rotation angle instead would not work and produces an entirely different gradient:

background: conic-gradient(white 45deg, black 225deg, white 405deg);

A conic gradient with a radial gradient overlaid on it, to draw a hue & saturation wheel:
background: radial-gradient(gray, transparent),
            conic-gradient(red, magenta, blue, aqua, lime, yellow, red);
border-radius: 50%;
width: 200px; height: 200px;

A conic gradient used to draw a simple pie chart. The 0deg color stop positions will be fixed up to be equal to the position of the color stop before them. This will produce infinitesimal (invisible) transitions between the color stops with different colors, effectively producing solid color segments.
background: conic-gradient(yellowgreen 40%, gold 0deg 75%, #f06 0deg);
border-radius: 50%;
width: 200px; height: 200px;

3.4. Repeating Gradients: the repeating-linear-gradient(), repeating-radial-gradient(), and repeating-conic-gradient() notations

In addition to linear-gradient(), radial-gradient(), and conic-gradient(), this specification defines repeating-linear-gradient(), repeating-radial-gradient(), and repeating-conic-gradient() values. These notations take the same values and are interpreted the same as their respective non-repeating siblings defined previously.

Basic repeating conic gradient:
background: repeating-conic-gradient(gold, #f06 20deg);

Repeating color stops with abrupt transitions creates a starburst-type background:
background: repeating-conic-gradient(
                hsla(0,0%,100%,.2) 0deg 15deg,
                hsla(0,0%,100%,0) 0deg 30deg
            ) #0ac;

Here repeating color stops with abrupt transitions are used to create a checkerboard:
background: repeating-conic-gradient(black 0deg 25%, white 0deg 50%);
background-size: 60px 60px;

The same checkerboard can be created via non-repeating conic gradients:

background: conic-gradient(black 25%, white 0deg 50%, black 0deg 75%, white 0deg);
background-size: 60px 60px;

3.5. Gradient Color-Stops

<color-stop-list> =
  [ <linear-color-stop> [, <linear-color-hint>]? ]# , <linear-color-stop>
<linear-color-stop> = <color> && <color-stop-length>
<linear-color-hint> = <length-percentage>
<color-stop-length> = <length-percentage>{1,2}

<angular-color-stop-list> =
  [ <angular-color-stop> [, <angular-color-hint>]? ]# , <angular-color-stop>
<angular-color-stop> = <color> && <color-stop-angle>?
<angular-color-hint> = <angle-percentage>
<color-stop-angle> = <angle-percentage>{1,2}

<color-stop> = <color-stop-length> | <color-stop-angle>
<color-stop> <color-stop> , , <color-hint> <color-hint> , , , , <color-stop> <color-stop>

Are lengths useful in <angular-color-stop>, for a given gradient circle?

This is past the complexity point where it can be easily understood with just prose. Add a diagram illustrating the possibilities, preferably for all three kinds of gradients (to show off the three shapes of gradient lines).

The colors in gradients are specified using color stops. A color stop is a combination of a color and one or two positions. (Depending on the type of gradient, that position can be a length, angle, or percentage.) While every color stop conceptually has at least one position, the position can be omitted in the syntax. (It gets automatically filled in by the user agent; see below for details.) The unit identifier may be omitted if the position is zero.

Between two color stops there can be a color interpolation hint, which specifies how the colors of the two color stops on either side should be interpolated in the space between them (by default, they interpolate linearly). There can only be at most one color interpolation hint between any two given color stops; using more than that makes the function invalid.

Color stops are organized into a color stop list, which is a list of one or more color stops. The first and last color stops in the list must have a color (though their position can be omitted).

Color stops and color hints are placed on a gradient line, which defines the colors at every point of a gradient. The gradient function defines the shape and length of the gradient line, along with its starting point and ending point.

Color stops and color hints must be specified in order. Percentages refer to the length of the gradient line between the starting point and ending point, 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 along the gradient line. Angles are measured with 0deg pointing up, and positive angles corresponding to clockwise rotations from there.

Color stops and color hints 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 or color hint can be placed at any position on the gradient line.

A color stop with two locations is mostly equivalent to specifying two color stops with the same color, one for each position. Specifying two locations makes it easier to create solid-color "stripes" in a gradient, without having to repeat the color twice.

The position of a color stop can be omitted. This causes the color stop to position itself automatically between the two surrounding stops. If multiple stops in a row lack a position, they space themselves out equally.

The following steps must be applied in order to process the <color-stop-list>. After applying these rules, all color stops and color hints will have a definite position and color (if appropriate) and they will be in ascending order:

  1. 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%.
  2. If a color stop or color hint has a position that is less than the specified position of any color stop or color hint before it in the list, set its position to be equal to the largest specified position of any color stop or color hint before it.
  3. 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.

This requires us to wait until after layout to do fix-up, because implied-position stops (set by step 3) may depend on stops that need layout information to place, and which may be corrected by step 2. Swapping steps 2 and 3 would let us interpolate color stops purely at computed-value time, which is a nice plus, at the cost of changing behavior from level 3 for some edge cases that triggered fixup. Make sure this is handled well in the serialization rules.

At each color stop position, the line is the color of the color stop. Between two color stops, the line’s color is interpolated between the colors of the two color stops, with the interpolation taking place in premultiplied RGBA space.

By default, this interpolation is linear—at 25%, 50%, or 75% of the distance between two color stops, the color is a 25%, 50%, or 75% blend of the colors of the two stops.

However, if a color hint was provided between two color stops, the interpolation is non-linear, and controlled by the hint:

  1. Determine the location of the color hint as a percentage of the distance between the two color stops, denoted as a number between 0 and 1, where 0 indicates the hint is placed right on the first color stop, and 1 indicates the hint is placed right on the second color stop. Let this percentage be H.
  2. For any given point between the two color stops, determine the point’s location as a percentage of the distance between the two color stops, in the same way as the previous step. Let this percentage be P.
  3. Let C, the color weighting at that point, be equal to PlogH(.5).
  4. The color at that point is then a linear blend between the colors of the two color stops, blending (1 - C) of the first stop and C of the second stop.

Note: If the hint is placed halfway between the two stops, this is thus the ordinary linear interpolation. If the hint is placed anywhere else, it dictates the position of the "halfway point", where the color is an equal blend between the two color stops, and produces smooth, even blends between the color stops and the "halfway point".

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.

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)

8. linear-gradient(red, 25%, white)
   =14=>
   linear-gradient(red 0%, rgb(100%,50%,50%) 25%, white 100%)
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:

(Image requires SVG)

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:

(Image requires SVG)

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.

4. Sizing Images and Objects in CSS

4.1. Sizing Objects: the object-fit property

Name: object-fit
Value: fill | none | [contain | cover] || scale-down
Initial: fill
Applies to: replaced elements
Inherited: no
Percentages: n/a
Media: visual
Computed value: specified value
Canonical order: per grammar
Animatable: no

The object-fit property specifies how the contents of a replaced element should be fitted to the box established by its used height and width.

fill
The replaced content is sized to fill the element’s content box: the object’s concrete object size is the element’s used width and height.
none
The replaced content is not resized to fit inside the element’s content box: determine the object’s concrete object size using the default sizing algorithm with no specified size, and a default object size equal to the replaced element’s used width and height.
contain
The replaced content is sized to maintain its aspect ratio while fitting within the element’s content box: its concrete object size is resolved as a contain constraint against the element’s used width and height.

If the scale-down flag is used, size the content as if none or contain were specified, whichever would result in a smaller concrete object size.

Note: Both none and contain respect the content’s intrinsic aspect ratio, so the concept of "smaller" is well-defined.

cover
The replaced content is sized to maintain its aspect ratio while filling the element’s entire content box: its concrete object size is resolved as a cover constraint against the element’s used width and height.

If the scale-down flag is used, size the content as if none or cover were specified, whichever would result in a smaller concrete object size.

Note: Both none and cover respect the content’s intrinsic aspect ratio, so the concept of "smaller" is well-defined.

scale-down
Equivalent to contain scale-down.

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 (shown with a green background), using the initial value for object-position. In this case, scale-down and scale-down contain would look identical to contain, and scale-down cover would look identical to none.

Note: The object-fit property has similar semantics to the fit attribute in [SMIL10] and the <meetOrSlice> parameter on the preserveAspectRatio attribute in [SVG11].

Note: Per the object size 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.

5. Image Processing

5.1. 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
Percentages: n/a
Media: visual
Computed value: as specified, except with <resolution> possibly altered by computed for snap (see below)
Canonical order: per grammar
Animatable: no

The image-set() notation can alter the intrinsic resolution of an image, which ideally would be automatically honored without having to set this property. How should we best address this? Change the initial value to auto, meaning "1dppx, unless CSS says otherwise"? Say that image-resolution has no effect on images whose resolution was set by something else in CSS? Or somehow wordsmithing image-set() in some way such that it always produces 1dppx images somehow?

The image-resolution property specifies the intrinsic resolution of all raster images used in or on the element. It affects both content images (e.g. replaced elements and generated content) and decorative images (such as background-image). The intrinsic resolution of an image is used to determine the image’s intrinsic dimensions. Values have the following meanings:

<resolution>
Specifies the intrinsic resolution explicitly. A "dot" in this case corresponds to a single image pixel.
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 1dppx.
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.

As vector formats such as SVG do not have an intrinsic resolution, this property has no effect on vector images.

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.

img { image-resolution: from-image 300dpi }
img { image-resolution: 300dpi from-image }
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:

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. Interpolation

Note: No change from [css3-images].

7. Serialization

Note: No change from [css3-images].

8. Privacy and Security Considerations

Note: No change from [css3-images].

9. Changes

Changes Since the 11 September 2012 Working Draft

Changes Since Level 3

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:

style sheet
A CSS style sheet.
renderer
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

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.

Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

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 property 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

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

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.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-BACKGROUNDS-3]
Bert Bos; Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 17 October 2017. CR. URL: https://www.w3.org/TR/css-backgrounds-3/
[CSS-COLOR-4]
Tab Atkins Jr.; Chris Lilley. CSS Color Module Level 4. 5 July 2016. WD. URL: https://www.w3.org/TR/css-color-4/
[CSS-LISTS-3]
Tab Atkins Jr.. CSS Lists and Counters Module Level 3. 20 March 2014. WD. URL: https://www.w3.org/TR/css-lists-3/
[CSS-UI-4]
Florian Rivoal. CSS Basic User Interface Module Level 4. 22 September 2015. WD. URL: https://www.w3.org/TR/css-ui-4/
[CSS-VALUES-3]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 29 September 2016. CR. URL: https://www.w3.org/TR/css-values-3/
[CSS21]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: https://www.w3.org/TR/CSS2
[CSS3-COLOR]
Tantek Çelik; Chris Lilley; David Baron. CSS Color Module Level 3. 7 June 2011. REC. URL: https://www.w3.org/TR/css3-color
[CSS3-IMAGES]
Elika Etemad; Tab Atkins Jr.. CSS Image Values and Replaced Content Module Level 3. 17 April 2012. CR. URL: https://www.w3.org/TR/css3-images/
[CSS3-TRANSFORMS]
Simon Fraser; et al. CSS Transforms Module Level 1. 26 November 2013. WD. URL: https://www.w3.org/TR/css-transforms-1/
[CSSOM-1]
Simon Pieters; Glenn Adams. CSS Object Model (CSSOM). 17 March 2016. WD. URL: https://www.w3.org/TR/cssom-1/
[HTML]
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[MEDIA-FRAGS]
Raphaël Troncy; et al. Media Fragments URI 1.0 (basic). 25 September 2012. REC. URL: https://www.w3.org/TR/media-frags/
[PNG]
Tom Lane. Portable Network Graphics (PNG) Specification (Second Edition). 10 November 2003. REC. URL: https://www.w3.org/TR/PNG
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[SELECT]
Tantek Çelik; et al. Selectors Level 3. 29 September 2011. REC. URL: https://www.w3.org/TR/css3-selectors/
[SELECTORS4]
Elika Etemad; Tab Atkins Jr.. Selectors Level 4. 2 May 2013. WD. URL: https://www.w3.org/TR/selectors4/
[SVG-INTEGRATION]
Cameron McCormack; Doug Schepers; Dirk Schulze. SVG Integration. 17 April 2014. WD. URL: https://www.w3.org/TR/svg-integration/
[SVG11]
Erik Dahlström; et al. Scalable Vector Graphics (SVG) 1.1 (Second Edition). 16 August 2011. REC. URL: https://www.w3.org/TR/SVG11/
[WebIDL]
Cameron McCormack; Boris Zbarsky; Tobie Langel. Web IDL. 15 December 2016. ED. URL: https://heycam.github.io/webidl/

Informative References

[SMIL10]
Philipp Hoschka. Synchronized Multimedia Integration Language (SMIL) 1.0 Specification. 15 June 1998. REC. URL: https://www.w3.org/TR/1998/REC-smil-19980615

Property Index

Name Value Initial Applies to Inh. %ages Media Ani­mat­able Canonical order Com­puted value
image-resolution [ from-image || <resolution> ] && snap? 1dppx all elements yes n/a visual no per grammar as specified, except with <resolution> possibly altered by computed for snap (see below)
object-fit fill | none | [contain | cover] || scale-down fill replaced elements no n/a visual no per grammar specified value

IDL Index

partial interface CSS {
  [SameObject] readonly attribute Map elementSources;
};

Issues Index

This solution assumes that resolution is a proxy for filesize, and therefore doesn’t appropriately handle multi-resolution sets of vector images, or mixing vector images with raster ones (e.g. for icons). For example, use a vector for high-res, pixel-optimized bitmap for low-res, and same vector again for low-bandwidth (because it’s much smaller, even though it’s higher resolution).
We should add "w" and "h" dimensions as a possibility, and a "format()" function, to match the functionality of HTML’s picture.
Is this restriction needed?
Do we need to be able to refer to elements in external documents (such as SVG paint servers)? Or is it enough to just use url() for this?
This name conflicts with a somewhat similar function in GCPM. This needs to be resolved somehow.
Want the ability to do "reflections" of an element, either as a background-image on the element or in a pseudo-element. This needs to be specially-handled to avoid triggering the cycle-detection.
When we have overflow:paged, how can we address a single page in the view?
Requiring some degree of stacking context on the element appears to be required for an efficient implementation. Do we need a full stacking context, or just a pseudo-stacking context? Should it need to be a stacking context normally, or can we just render it as a stacking context when rendering it to element()?
This reuse of the ID selector matches Moz behavior. I’m trying to avoid slapping a <custom-ident> right in the beginning of the grammar, as that eats too much syntax-space. Another possibility, though, is to start the value with a language-defined keyword followed by a <custom-ident>, like element(external fancy) or something. Naming suggestions welcome.
Usually in conic gradients the sharp transition at 0deg is undesirable, which is typically avoided by making sure the first and last color stops are the same color. Perhaps it would be useful to have a keyword for automatically achieving this.
Would a radius (inner & outer) for clipping the gradient be useful? If so, we could also support lengths in color stop positions, since we now have a specific radius.
Are elliptical conic gradients useful? Do graphics libraries support them?
Are lengths useful in <angular-color-stop>, for a given gradient circle?
This is past the complexity point where it can be easily understood with just prose. Add a diagram illustrating the possibilities, preferably for all three kinds of gradients (to show off the three shapes of gradient lines).
This requires us to wait until after layout to do fix-up, because implied-position stops (set by step 3) may depend on stops that need layout information to place, and which may be corrected by step 2. Swapping steps 2 and 3 would let us interpolate color stops purely at computed-value time, which is a nice plus, at the cost of changing behavior from level 3 for some edge cases that triggered fixup. Make sure this is handled well in the serialization rules.
The image-set() notation can alter the intrinsic resolution of an image, which ideally would be automatically honored without having to set this property. How should we best address this? Change the initial value to auto, meaning "1dppx, unless CSS says otherwise"? Say that image-resolution has no effect on images whose resolution was set by something else in CSS? Or somehow wordsmithing image-set() in some way such that it always produces 1dppx images somehow?