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…”.
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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]
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:
A style sheet is conformant to this specification
if all of its statements that use syntax defined in this module are valid
according to the generic CSS grammar and the individual grammars of each
feature defined in this module.
A renderer is conformant to this specification
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by this specification by parsing them correctly
and rendering the document accordingly. However, the inability of a
UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.)
An authoring tool is conformant to this specification
if it writes style sheets that are syntactically correct according to the
generic CSS grammar and the individual grammars of each feature in
this module, and meet all other conformance requirements of style sheets
as described in this module.
Partial implementations
So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers must treat as invalid (and ignore
as appropriate) any at-rules, properties, property values, keywords,
and other syntactic constructs for which they have no usable level of
support. In particular, user agents must not selectively
ignore unsupported component values and honor supported values in a single
multi-value property declaration: if any value is considered invalid
(as unsupported values must be), CSS requires that the entire declaration
be ignored.
Implementations of Unstable and Proprietary Features
Once a specification reaches the Candidate Recommendation stage,
non-experimental implementations are possible, and implementors should
release an unprefixed implementation of any CR-level feature they
can demonstrate to be correctly implemented according to spec.
To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental
CSS renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before
releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS
Working Group.
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. ↵