This module contains the features of CSS relating to the backgrounds of boxes on the page.
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-backgrounds” in the title, like this:
“[css-backgrounds] …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 module is currently maintained as a diff against
the parts related to backgrounds of
CSS Backgrounds and Borders Module Level 3 [CSS3BG].
We will fold in the text once it’s all formatted up and in CR again,
as this will reduce the effort of keeping them in sync
(source diffs will be accurate in reflecting the differences).
If only one value is given,
and that value is start or end,
then the keyword is duplicated;
otherwise the second keyword defaults to center.
The resulting value is treated as a two-component value.
The background is painted within (clipped to)
the specified box of the element.
text
The background is painted within (clipped to)
the intersection of the border box
and the geometry of the text in the element and its in-flow and floated descendants.
border
The background is clipped to the area painted by the border, taking border-width and border-style into account but ignoring any transparency introduced by border-color.
2.3. Background Image Layers: the background-tbd shorthand property
The background-tbd property is a shorthand property that sets all the same properties as the background shorthand
except for background-color,
allowing authors to easily declare and position background images
while letting background-color cascade through independently.
The name of this property is discussed in issue 9083.
This example sets two background layers later in the cascade.
By using background-tbd, the previously set background-color won’t be overridden.
p {background-color: green;}
p {background-tbd:url(a.png) top left,url(b.png) top left no-repeat;}
This example tries to set the background color in addition to
the background image. But for that to work, background needs to be used instead of background-tbd.
So the background-tbd declaration will be dropped.
p {background:url(pass.png) green;/* valid */
background-tbd: url(fail.png) red;/* invalid */}
In addition to the many contributors to the [CSS1], [CSS21],
and [CSS3BG] predecessors to this module,
the editors would like to thank
Tab Atkins,
and Håkon Wium Lie
for their suggestions and feedback specifically for this Level 4.
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.
This module is currently maintained as a diff against
the parts related to backgrounds of
CSS Backgrounds and Borders Module Level 3 [CSS3BG].
We will fold in the text once it’s all formatted up and in CR again,
as this will reduce the effort of keeping them in sync
(source diffs will be accurate in reflecting the differences). ↵
Specify the value assignment in more detail. Should expand just like Level 3. ↵
Specify what happens to set of properties that are not set. Maybe they’re just not set? ↵
This section is still being worked out. The tricky thing is making all the start/end keywords work sanely. ↵
Or should this be defining the -webkit-background-clip property,
saying that all the values are identical,
with this additional text value? ↵
The name of this property is discussed in issue 9083. ↵
Firefox49+Safari1+Chrome1+Opera?Edge79+Edge (Legacy)12+IE6+Firefox for Android?iOS Safari?Chrome for Android?Android WebView37+Samsung Internet?Opera Mobile18+
Firefox49+Safari1+Chrome1+Opera?Edge79+Edge (Legacy)12+IE6+Firefox for Android?iOS Safari?Chrome for Android?Android WebView37+Samsung Internet?Opera Mobile?