CSS Images Module Level 5

Editor’s Draft,

More details about this document
This version:
https://drafts.csswg.org/css-images-5/
Latest published version:
https://www.w3.org/TR/css-images-5/
Feedback:
CSSWG Issues Repository
Inline In Spec
Editors:
Tab Atkins Jr. (Google)
Elika J. Etemad / fantasai (Invited Expert)
Lea Verou (Invited Expert)
Suggest an Edit for this Spec:
GitHub Editor

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 [CSS2] and in the previous level of this specification [css-images-4].

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

Status of this document

This 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.

Please send feedback by filing issues in GitHub (preferred), including the spec code “css-images” in the title, like this: “[css-images] …summary of comment…”. All issues and comments are archived. Alternately, feedback can be sent to the (archived) public mailing list www-style@w3.org.

This document is governed by the 2 November 2021 W3C Process Document.

1. Introduction

This is a diff spec over [css-images-4].

2. Sizing Images and Objects in CSS

2.1. Setting The Viewbox: the object-view-box property

Name: object-view-box
Value: none | <basic-shape-rect>
Initial: none
Applies to: replaced elements
Inherited: no
Percentages: n/a
Computed value: specified keyword, or computed function
Canonical order: per grammar
Animation type: as if possible, otherwise discrete

The object-view-box property specifies a "view box" over an element, which allows zooming or panning over the element’s contents. It maps to the <svg viewBox> attribute in SVG. [SVG2]

Make sure behavior is properly consistent for SVG.

none

The element does not have a view box.

<basic-shape-rect>

If the element does not have both a natural width and a natural height, this value has no effect, similar to none.

Otherwise, specifies a view box for the element.

First, resolve the <basic-shape-rect> against a reference box formed by the element’s natural sizes to obtain the element’s view box.

For all purposes, the element is now treated as having natural sizes equal to the view box’s width and height. If the element had a natural aspect ratio, it’s now treated as instead having the same ratio as the view box. Further adjustments to the size/position of the element’s contents, such as object-position or object-fit, are similarly performed on the view box instead.

When the element is painted, its contents are scaled and translated such that the element’s contents retain the same position and size, relative to the view box’s final size and position, that they had when the view box was determined (above).

Have not yet defined what happens if the view box is zero-area. It’s an error case, so precise behavior isn’t important; just need to see what impls want to do about it.

Note: Some replaced elements might have a built-in notion of a "view box", such as the svg element. Unless otherwise specified, this property does not interact with such notions; the built-in notion has its effect as normal, producing a replaced element with natural sizes, then this property applies on top of that.

Issue: example here. Need a basic one showing off zooming in, one showing off zooming out, and one showing off interaction with object-fit to display how the parts outside the viewbox can still be painted.

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.

Tests

Tests relating to the content of this specification may be documented in “Tests” blocks like this one. Any such block is non-normative.


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.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

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.

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.

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-DISPLAY-4]
CSS Display Module Level 4 URL: https://drafts.csswg.org/css-display-4/
[CSS-IMAGES-3]
Tab Atkins Jr.; Elika Etemad; Lea Verou. CSS Images Module Level 3. URL: https://w3c.github.io/csswg-drafts/css-images-3/
[CSS-IMAGES-4]
Tab Atkins Jr.; Elika Etemad; Lea Verou. CSS Image Values and Replaced Content Module Level 4. URL: https://w3c.github.io/csswg-drafts/css-images-4/
[CSS-SHAPES-1]
Rossen Atanassov; Alan Stearns. CSS Shapes Module Level 1. URL: https://w3c.github.io/csswg-drafts/css-shapes/
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. URL: https://w3c.github.io/csswg-drafts/css-values-4/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://datatracker.ietf.org/doc/html/rfc2119
[SVG2]
Amelia Bellamy-Royds; et al. Scalable Vector Graphics (SVG) 2. URL: https://svgwg.org/svg2-draft/

Informative References

[CSS2]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. URL: https://w3c.github.io/csswg-drafts/css2/

Property Index

Name Value Initial Applies to Inh. %ages Anim­ation type Canonical order Com­puted value
object-view-box none | <basic-shape-rect> none replaced elements no n/a as if possible, otherwise discrete per grammar specified keyword, or computed function

Issues Index

Make sure behavior is properly consistent for SVG.
Have not yet defined what happens if the view box is zero-area. It’s an error case, so precise behavior isn’t important; just need to see what impls want to do about it.