CSS Conditional Rules Module Level 3

CSS Conditional Rules Module Level 3
CSS Conditional Rules Module Level 3
W3C Candidate Recommendation
4 April
2013
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Abstract
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 for conditional
processing of parts of style sheets, conditioned on capabilities of the
processor or the document the style sheet is being applied to. 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 allowing nesting of certain at-rules
inside ‘
@media
’, and the addition of the
@supports
’ rule for conditional processing.
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/.
This document was produced by the
CSS Working Group
as a
Candidate
Recommendation.
A Candidate Recommendation is a document that has been widely reviewed
and is ready for implementation. W3C encourages everybody to implement
this specification and return comments to the (
archived
) public
mailing list
www-style@w3.org
(see
instructions
). When sending
e-mail, please put the text “css3-conditional” in the subject,
preferably like this: “[
css3-conditional
…summary
of comment…
Publication as a Candidate Recommendation 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.
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
See the section
“CR Exit Criteria”
for details on advancing this specification to W3C Recommendation. The
specification will remain Candidate Recommendation at least until 2
October 2013. A
test
suite and implementation report
are under development.
See the section
“Changes”
for changes since
the last Working Draft.
The following features are at risk:
The inclusion of ‘
@font-face
’ rules and
@keyframes
’ rules as allowed within all of
the @-rules in this specification is at risk, though only because of the
relative rates of advancement of specifications. If this specification is
able to advance faster than one or both of the specifications defining
those rules, then the inclusion of those rules will move from this
specification to the specification defining those rules.
The addition of support for @-rules inside of conditional grouping
rules is at risk; if interoperable implementations are not found, it may
be removed to advance the other features in this specification to
Proposed Recommendation.
The ‘
@supports
’ rule is at risk; if
interoperable implementations are not found, it may be removed to advance
the other features in this specification to Proposed Recommendation.
Table of contents
1.
Introduction
1.1.
Background
1.2.
Module
Interactions
1.3.
Document
Conventions
2.
Processing of
conditional group rules
3.
Contents of
conditional group rules
4.
Placement of conditional
group rules
5.
Media-specific style
sheets: the ‘
@media
’ rule
6.
Feature queries:
the ‘
@supports
’ rule
6.1.
Definition of support
7.
APIs
7.1.
Extensions to the
CSSRule
interface
7.2.
The
CSSGroupingRule
interface
7.3.
The
CSSConditionRule
interface
7.4.
The
CSSMediaRule
interface
7.5.
The
CSSSupportsRule
interface
7.6.
The
CSS
interface, and the
supports()
function
Grammar
8.
Conformance
8.1.
Base
Modules
8.2.
Conformance Classes
8.3.
Partial
Implementations
8.4.
Experimental
Implementations
8.5.
CR Exit
Criteria
9.
Changes
Acknowledgments
References
Normative
references
Other references
Index
1.
Introduction
1.1.
Background
This section is not normative.
[CSS21]
defines
one type of conditional group rule, the ‘
@media
’ rule, and allows only rulesets (not other
@-rules) inside of it. The ‘
@media
’ rule
provides the ability to have media-specific style sheets, which is also
provided by style sheet linking features such as ‘
@import
’ and

. The
restrictions on the contents of ‘
@media
’ rules
made them less useful; they have forced authors using CSS features
involving @-rules in media-specific style sheets to use separate style
sheets for each medium.
This specification extends the rules for the contents of conditional
group rules to allow other @-rules, which enables authors to combine CSS
features involving @-rules with media specific style sheets within a
single style sheet.
This specification also defines an additional type of conditional group
rule, ‘
@supports
’, to address author and user
requirements.
The ‘
@supports
’ rule allows CSS to be
conditioned on implementation support for CSS properties and values. This
rule makes it much easier for authors to use new CSS features and provide
good fallback for implementations that do not support those features. This
is particularly important for CSS features that provide new layout
mechanisms, and for other cases where a set of related styles needs to be
conditioned on property support.
1.2.
Module Interactions
This module replaces and extends the ‘
@media
rule feature defined in
[CSS21]
section
7.2.1
and
incorporates the modifications previously made non-normatively by
[MEDIAQ]
section
Its current definition depends on @-rules defined in
[CSS3-FONTS]
and
[CSS3-ANIMATIONS]
, but that
dependency is only on the assumption that those modules will advance ahead
of this one. If this module advances faster, then the dependency will be
reversed.
1.3.
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.
2.
Processing of conditional
group rules
This specification defines some CSS @-rules, called
conditional group rules
, that associate a
condition with a group of other CSS rules. These different rules allow
testing different types of conditions, but share common behavior for how
their contents are used when the condition is true and when the condition
is false.
For example, this rule:
@media print {
/* hide navigation controls when printing */
#navigation { display: none }
causes a particular CSS rule (making elements with ID "navigation" be
display:none) apply only when the style sheet is used for a print medium.
Each conditional group rule has a condition, which at any time evaluates
to true or false. When the condition is true, CSS processors
must
apply the rules inside the group rule as though they
were at the group rule's location; when the condition is false, CSS
processors
must not
apply any of rules inside the group
rule. The current state of the condition does not affect the CSS object
model, in which the contents of the group rule always remain within the
group rule.
This means that when multiple conditional group rules are nested, a rule
inside of both of them applies only when all of the rules' conditions are
true.
For example, with this set of nested rules:
@media print { // rule (1)
/* hide navigation controls when printing */
#navigation { display: none }
@media (max-width: 12cm) { // rule (2)
/* keep notes in flow when printing to narrow pages */
.note { float: none }
the condition of the rule marked (1) is true for print media, and the
condition of the rule marked (2) is true when the width of the display
area (which for print media is the page box) is less than or equal to
12cm. Thus the rule ‘
#navigation { display: none
’ applies whenever this style sheet is applied to print media,
and the rule ‘
.note { float: none }
’ is applied
only when the style sheet is applied to print media
and
the width
of the page box is less than or equal to 12 centimeters.
When the condition for a conditional group rule changes, CSS processors
must
reflect that the rules now apply or no longer apply,
except for properties whose definitions define effects of computed values
that persist past the lifetime of that value (such as for some properties
in
[CSS3-TRANSITIONS]
and
[CSS3-ANIMATIONS]
).
3.
Contents of conditional
group rules
The syntax of each conditional group rule consists of some syntax
specific to the type of rule followed by a
group
rule body
, which is a block (pair of braces) containing a sequence
of rules.
A group rule body is allowed to contain rulesets and any @-rules that
are allowed at the top level of a style sheet before and after a ruleset.
This means that @-rules that must occur at the beginning of the style
sheet (such as ‘
@charset
’, ‘
@import
’, and ‘
@namespace
rules) are not allowed inside of conditional group rules. Conditional
group rules can be nested.
In terms of the grammar, this specification defines the following
productions for use in the grammar of conditional group rules:
nested_statement
: ruleset |
media
| page | font_face_rule | keyframes_rule |
supports_rule
group_rule_body
: '{' S*
nested_statement
* '}' S*
in which all the productions are defined in that grammar with the
exception of
font_face_rule
defined in
[CSS3-FONTS]
keyframes_rule
defined in
[CSS3-ANIMATIONS]
, and
media
and
supports_rule
defined in this
specification.
In general, future CSS specifications that add new @-rules that are not
forbidden to occur after some other types of rules should modify this
nested_statement
production to
keep the grammar accurate.
Style sheets
must not
use rules other than the allowed
ones inside conditional group rules.
CSS processors
must
ignore rules that are not allowed
within a group rule, and
must
handle invalid rules inside
of group rules as described in
section 4.2
(Rules for handling parsing errors)
section 4.1.5
(At-rules)
, and
section 4.1.7
(Rule sets, declaration blocks, and selectors)
of
[CSS21]
4.
Placement of conditional group rules
Conditional group rules are allowed at the top-level of a style sheet,
and inside other conditional group rules. CSS processors
must
process such rules as
described above
Any rules that are not allowed after a ruleset (e.g., ‘
@charset
’, ‘
@import
’, or
@namespace
’ rules) are also not allowed after
a conditional group rule. Therefore, style sheets
must
not
place such rules after a conditional group rules, and CSS
processors
must
ignore such rules.
5.
Media-specific style sheets:
the ‘
@media
’ rule
The
@media
’ rule
is a conditional group rule whose condition is a media query. It consists
of the at-keyword ‘
@media
’ followed by a
(possibly empty) media query list (as defined in
[MEDIAQ]
), followed by a group rule
body. The condition of the rule is the result of the media query.
This ‘
@media
’ rule:
@media screen and (min-width: 35em),
print and (min-width: 40em) {
#section_navigation { float: left; width: 10em; }
has the condition ‘
screen and (min-width: 35em),
print and (min-width: 40em)
’, which is true for screen displays
whose viewport is at least 35 times the initial font size and for print
displays whose viewport is at least 40 times the initial font size. When
either of these is true, the condition of the rule is true, and the rule
#section_navigation { float: left; width: 10em;
’ is applied.
In terms of the grammar, this specification extends the
media
production in the
Grammar of CSS 2.1
[CSS21]
, Appendix G)
into:
media
: MEDIA_SYM S* media_query_list
group_rule_body
where the
group_rule_body
production is defined in this specification, the
media_query_list
production is defined in
[MEDIAQ]
, and the others are defined
in the
Grammar of CSS
2.1
[CSS21]
Appendix G).
6.
Feature queries: the
@supports
’ rule
The
@supports
rule
is a conditional group rule whose condition tests whether the
user agent supports CSS property:value pairs. Authors can use it to write
style sheets that use new features when available but degrade gracefully
when those features are not supported. CSS has existing mechanisms for
graceful degradation, such as ignoring unsupported properties or values,
but these are not always sufficient when large groups of styles need to be
tied to the support for certain features, as is the case for use of new
layout system features.
The syntax of the condition in the ‘
@supports
’ rule is slightly more complicated than for
the other conditional group rules (though has some similarities to media
queries) since:
negation is needed so that the new-feature styles and the fallback
styles can be separated (within the forward-compatible grammar's rules
for the syntax of @-rules), and not required to override each other
conjunction (and) is needed so that multiple required features can be
tested
disjunction (or) is needed when there are multiple alternative
features for a set of styles, particularly when some of those
alternatives are vendor-prefixed properties or values
Therefore, the syntax of the ‘
@supports
’ rule
allows testing for property:value pairs, and arbitrary conjunctions (and),
disjunctions (or), and negations (not) of them.
This extends the lexical scanner in the
Grammar of CSS 2.1
[CSS21]
, Appendix G) by
adding:
@{S}{U}{P}{P}{O}{R}{T}{S} {return
SUPPORTS_SYM
;}
{O}{R} {return
OR
;}
This then extends the grammar in the
Grammar of CSS 2.1
using the lexical scanner there, with the additions of
AND
and
NOT
tokens defined in the Media Queries specification
[MEDIAQ]
and the
OR
and
SUPPORTS_SYM
tokens defined above,
and with
declaration
any
and
unused
productions and the
FUNCTION
token taken from the core syntax of CSS defined in
section 4.1.1
(Tokenization)
of
[CSS21]
, by adding:
supports_rule
SUPPORTS_SYM
supports_condition
group_rule_body
supports_condition
supports_negation
supports_conjunction
supports_disjunction
supports_condition_in_parens
supports_condition_in_parens
: ( '('
supports_condition
* ')' ) |
supports_declaration_condition
general_enclosed
supports_negation
NOT
supports_condition_in_parens
supports_conjunction
supports_condition_in_parens
AND
supports_condition_in_parens
)+
supports_disjunction
supports_condition_in_parens
OR
supports_condition_in_parens
)+
supports_declaration_condition
: '('
declaration
')'
general_enclosed
: (
FUNCTION
| '(' ) (
any
unused
)* ')'
Implementations
must
parse ‘
@supports
’ rules based on the above grammar, and when
interpreting the above grammar,
must
match the production
before an
operator in preference to the one after it.
The above grammar is purposely very loose for forwards-compatibility
reasons, since the
general_enclosed
production
allows for substantial future extensibility. Any ‘
@supports
’ rule that does not parse according to the
grammar above (that is, a rule that does not match this loose grammar
which includes the
general_enclosed
production) is
invalid. Style sheets
must not
use such a rule and
processors
must
ignore such a rule (including all of its
contents).
Each of these grammar terms is associated with a boolean result, as
follows:
supports_condition
The result is the result of the single child term.
supports_condition_in_parens
The result is the result of the single
supports_condition
or
supports_declaration_condition
child term.
supports_negation
The result is the
negation
of the result of the
supports_condition_in_parens
child term.
supports_conjunction
The result is true if the result of
all
of the
supports_condition_in_parens
child terms is true; otherwise it is false.
supports_disjunction
The result is true if the result of
any
of the
supports_condition_in_parens
child terms is true; otherwise it is false.
supports_declaration_condition
The result is whether the CSS processor
supports
the declaration within the
parentheses.
general_enclosed
The result is always false. Additionally, style sheets
must
not
write ‘
@supports
’ rules that
match this grammar production. (In other words, this production exists
only for future extensibility, and is not part of the description of a
valid style sheet in this level of the specification.)
Note that future levels may define functions or other
parenthesized expressions that can evaluate to true.
The condition of the ‘
@supports
’ rule is the
result of the
supports_condition
term that
is a child of the
supports_rule
term.
For example, the following rule
@supports ( display: flexbox ) {
body, #navigation, #content { display: flexbox; }
#navigation { background: blue; color: white; }
#article { background: white; color: black; }
applies the rules inside the ‘
@supports
rule only when ‘
display: flexbox
’ is
supported.
The following example shows an additional ‘
@supports
’ rule that can be used to provide an
alternative for when ‘
display: flexbox
’ is not
supported:
@supports not ( display: flexbox ) {
body { width: 100%; height: 100%; background: white; color: black; }
#navigation { width: 25%; }
#article { width: 75%; }
Note that the ‘
width
’ declarations may
be harmful to the flexbox-based layout, so it is important that they be
present only in the non-flexbox styles.
The following example checks for support for the ‘
box-shadow
’ property, including checking for
support for vendor-prefixed versions of it. When the support is present,
it specifies both ‘
box-shadow
’ (with the
prefixed versions) and ‘
color
’ in a way
what would cause the text to become invisible were ‘
box-shadow
’ not supported.
@supports ( box-shadow: 2px 2px 2px black ) or
( -moz-box-shadow: 2px 2px 2px black ) or
( -webkit-box-shadow: 2px 2px 2px black ) or
( -o-box-shadow: 2px 2px 2px black ) {
.outline {
color: white;
-moz-box-shadow: 2px 2px 2px black;
-webkit-box-shadow: 2px 2px 2px black;
-o-box-shadow: 2px 2px 2px black;
box-shadow: 2px 2px 2px black; /* unprefixed last */
To avoid confusion between ‘
and
’ and ‘
or
’, the syntax requires that both
and
’ and ‘
or
’ be specified explicitly (rather than, say,
using commas or spaces for one of them). Likewise, to avoid confusion
caused by precedence rules, the syntax does not allow ‘
and
’, ‘
or
’, and ‘
not
’ operators
to be mixed without a layer of parentheses.
For example, the following rule is not valid:
@supports (transition-property: color) or
(animation-name: foo) and
(transform: rotate(10deg)) {
// ...
Instead, authors must write one of the following:
@supports ((transition-property: color) or
(animation-name: foo)) and
(transform: rotate(10deg)) {
// ...
@supports (transition-property: color) or
((animation-name: foo) and
(transform: rotate(10deg))) {
// ...
Furthermore, whitespace is required after a ‘
not
’ and on both sides of an ‘
and
’ or ‘
or
’.
The declaration being tested must always occur within parentheses, when
it is the only thing in the expression.
For example, the following rule is not valid:
@supports display: flexbox {
// ...
Instead, authors must write:
@supports (display: flexbox) {
// ...
The syntax allows extra parentheses when they are not needed. This
flexibility is sometimes useful for authors (for example, when commenting
out parts of an expression) and may also be useful for authoring tools.
For example, authors may write:
@supports ((display: flexbox)) {
// ...
A trailing ‘
!important
’ on a declaration
being tested is allowed, though it won't change the validity of the
declaration.
For example, the following rule is valid:
@supports (display: flexbox !important) {
// ...
6.1.
Definition of
support
For forward-compatibility,
section 4.1.8
(Declarations and properties)
of
[CSS21]
defines rules for handling
invalid properties and values. CSS processors that do not implement or
partially implement a specification
must
treat any part
of a value that they do not implement, or do not have a usable level of
support for, as invalid according to this rule for handling invalid
properties and values, and therefore
must
discard the
declaration as a parse error.
A CSS processor is considered to
support
declaration (consisting of a property and value) if it accepts that
declaration (rather than discarding it as a parse error). If a processor
does not implement, with a usable level of support, the value given, then
it
must not
accept the declaration or claim support for
it.
Note that properties or values whose support is effectively
disabled by user preferences are still considered as supported by this
definition. For example, if a user has enabled a high-contrast mode that
causes colors to be overridden, the CSS processor is still considered to
support the ‘
color
’ property even though
declarations of the ‘
color
’ property may
have no effect. On the other hand, a developer-facing preference whose
purpose is to enable or disable support for an experimental CSS feature
does affect this definition of support.
These rules (and the equivalence between them) allow authors to use
fallback (either in the
[CSS1]
sense of declarations that are
overridden by later declarations or with the new capabilities provided by
the ‘
@supports
’ rule in this specification)
that works correctly for the features implemented. This applies especially
to compound values; implementations must implement all parts of the value
in order to consider the declaration supported, either inside a ruleset or
in the declaration condition of an ‘
@supports
rule.
7.
APIs
7.1.
Extensions to the
CSSRule
interface
The
CSSRule
interface is extended as follows:
partial interface CSSRule {
const unsigned short SUPPORTS_RULE = 12;
7.2.
The
CSSGroupingRule
interface
The
CSSGroupingRule
interface
represents an at-rule that contains other rules nested inside itself.
interface CSSGroupingRule : CSSRule {
readonly attribute
CSSRuleList
cssRules;
unsigned long insertRule (DOMString rule, unsigned long index);
void deleteRule (unsigned long index);
cssRules
of type
CSSRuleList
readonly
The
cssRules
attribute must return a
CSSRuleList
object for the list of CSS rules nested inside
the grouping rule.
insertRule(DOMString rule, unsigned long index)
, returns
unsigned long
The
insertRule
operation must insert a CSS rule
rule
into the CSS rule list returned by
cssRules
such that the inserted rule will be at position
index
, and any
rules previously at
index
or higher will increase their index
by one. It must throw INDEX_SIZE_ERR if index is greater than
cssRules.length
. It must throw SYNTAX_ERR if the rule has a
syntax error and is unparseable; this does not include syntax errors
handled by error handling rules for constructs inside of the rule, but
this does include cases where the string given does not parse into a
single CSS rule (such as when the string is empty) or where there is
anything other than whitespace or comments after that single CSS rule. It
must throw HIERARCHY_REQUEST_ERR if the rule cannot be inserted at the
location specified, for example, if an ‘
@import
’ rule is inserted inside a group rule.
The return value is the
index
parameter.
deleteRule (unsigned long index)
, return
void
The
deleteRule
operation must remove a CSS rule from the
CSS rule list returned by
cssRules
at
index
. It
must throw INDEX_SIZE_ERR if index is greater than or equal to
cssRules.length
7.3.
The
CSSConditionRule
interface
The
CSSConditionRule
interface represents all the "conditional" at-rules, which consist of a
condition and a statement block.
interface CSSConditionRule : CSSGroupingRule {
attribute DOMString conditionText;
conditionText
of type
DOMString
The
conditionText
attribute represents the condition of
the rule. Since what this condition does varies between the derived
interfaces of
CSSConditionRule
, those
derived interfaces may specify different behavior for this attribute
(see, for example,
CSSMediaRule
below). In the absence of such rule-specific behavior, the following
rules apply:
The
conditionText
attribute, on getting, must return the
result of serializing the associated condition.
On setting the
conditionText
attribute these steps must
be run:
Trim the given value of white space.
If the given value matches the grammar of the appropriate condition
production for the given rule, replace the associated CSS condition
with the given value.
Otherwise, do nothing.
7.4.
The
CSSMediaRule
interface
The
CSSMediaRule
interface
represents a ‘
@media
’ rule:
interface CSSMediaRule : CSSConditionRule {
readonly attribute
MediaList
media;
media
of type
MediaList
readonly
The
media
attribute must return a
MediaList
object for the list of media queries specified
with the ‘
@media
’ rule.
conditionText
of type
DOMString
(CSSMediaRule-specific definition for attribute on CSSConditionRule)
The
conditionText
attribute (defined on the
CSSConditionRule
parent rule),
on getting, must return the value of
media.mediaText
on the
rule.
Setting the
conditionText
attribute must set the
media.mediaText
attribute on the rule.
7.5.
The
CSSSupportsRule
interface
The
CSSSupportsRule
interface
represents a ‘
@supports
’ rule.
interface CSSSupportsRule : CSSConditionRule {
conditionText
of type
DOMString
(CSSSupportsRule-specific definition for attribute on CSSConditionRule)
The
conditionText
attribute (defined on the
CSSConditionRule
parent rule),
on getting, must return the condition that was specified, without any
logical simplifications, so that the returned condition will evaluate to
the same result as the specified condition in any conformant
implementation of this specification (including implementations that
implement future extensions allowed by the
general_enclosed
exensibility
mechanism in this specification). In other words, token stream
simplifications are allowed (such as reducing whitespace to a single
space or omitting it in cases where it is known to be optional), but
logical simplifications (such as removal of unneeded parentheses, or
simplification based on evaluating results) are not allowed.
7.6.
The
CSS
interface, and the
supports()
function
The
CSS
interface holds useful
CSS-related functions that do not belong elsewhere.
interface CSS {
static boolean supports(DOMString property, DOMString value);
static boolean supports(DOMString conditionText);
supports(DOMString property, DOMString value)
, returns
boolean
supports(DOMString conditionText)
, returns
boolean
When the
supports()
method is invoked with two
arguments
property
and
value
, it must return
true
if
property
is a literal match for the name
of a CSS property that the UA supports, and
value
would be
successfully parsed as a supported value for that property. (Literal
match means that no CSS escape processing is performed, and leading and
trailing whitespace are not stripped, so any leading whitespace, trailing
whitespace, or CSS escapes equivalent to the name of a property would
cause the method to return
false
.) Otherwise, it must return
false
When invoked with a single
conditionText
argument, it must
return
true
if
conditionText
, when parsed and
evaluated as a
supports_condition
, would
return true. Otherwise, it must return
false
Grammar
In order to allow these new @-rules in CSS style sheets, this
specification modifies the
stylesheet
production in the
Appendix G
grammar of
[CSS21]
by replacing
the
media
production defined in
[CSS21]
with the
media
production defined in this one, and
additionally inserting
| supports_rule
alongside
ruleset | media | page
8.
Conformance
8.1.
Base Modules
This specification defines conformance in terms of base modules, which
are modules that this specification builds on top of. The base modules of
this module are:
[CSS21]
All of the conformance requirements of all base modules are incorporated
as conformance requirements of this module, except where overridden by
this module.
Additionally, all conformance requirements related to validity of syntax
in this module and all of its base modules are to be interpreted as though
all syntax in all of those modules is valid.
For example, this means that grammar presented in modules other than
[CSS21]
must obey the
requirements that
[CSS21]
defines for the parsing of
properties, and that requirements for handling invalid syntax in
[CSS21]
do not treat
syntax added by other modules as invalid.
Additionally, the set of valid syntax can be increased by the
conformance of a style sheet or processor to additional modules; use of
such syntax does not make a style sheet nonconformant and failure to treat
such syntax as invalid does not make a processor nonconformant.
8.2.
Conformance
Classes
Conformance to the CSS Conditional Rules Module is defined for three
conformance classes:
style
sheet
CSS
style sheet
processor
A tool that reads CSS style sheets: it may be a renderer or
user-agent
that interprets the semantics of a style sheet and renders documents that
use style sheets, or it may be a validator that checks style sheets.
authoring tool
A tool that writes a style sheet.
A style sheet is conformant to the CSS Conditional Rules Module if it
meets all of the conformance requirements in the module that are described
as requirements of style sheets.
A processor is conformant to the CSS Conditional Rules Module if it
meets all applicable conformance requirements in the module that are
described as requirements of processors. In general, all requirements are
applicable to renderers. Requirements concerning a part of CSS not
performed by a processor are not applicable, e.g., requirements related to
rendering are not applicable to a validator. The inability of a processor
to correctly render a document due to limitations of the device does not
make it non-conformant. (For example, a renderer is not required to render
color on a monochrome monitor.)
An authoring tool is conformant to the CSS Conditional Rules Module if
it writes style sheets that conform to the module and (if it reads CSS) it
is a conformant processor.
8.3.
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.
8.4.
Experimental
Implementations
To avoid clashes with future CSS features, the CSS specifications
reserve a
prefixed
syntax
for proprietary property and value extensions to CSS. The CSS
Working Group recommends that experimental implementations of features in
CSS Working Drafts also use vendor-prefixed property or value names. This
avoids any incompatibilities with future changes in the draft. Once a
specification reaches the Candidate Recommendation stage, implementors
should implement the non-prefixed syntax for any feature they consider to
be correctly implemented according to spec.
8.5.
CR Exit Criteria
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.
9.
Changes
The following (non-editorial) changes were made to this specification
since the
13 December
2012 Working Draft
Require whitespace around ‘
and
’ and ‘
or
’ and after ‘
not
’.
Add note explaining that user preferences that effectively disable a
property (e.g., high-contrast mode disabling colors) do not effect the
definition of support.
Describe requirements for conditionText getter on CSSSupportsRule.
Clarify the definition of "literal match" in CSS.supports().
Specify behavior of CSSGroupingRule.insertRule when given an empty
string or more than one syntactically valid rule.
Acknowledgments
Thanks to the ideas and feedback from Tab Atkins, Arthur Barstow, Ben
Callahan,
Tantek Çelik
, Alex Danilo, Elika Etemad,
Pascal Germroth,
Björn Höhrmann
, Paul Irish,
Anne van Kesteren
, Vitor Menezes, Alex Mogilevsky, Chris
Moschini, James Nurthen, Simon Pieters,
Florian
Rivoal
Simon Sapin
, Nicholas Shanks, Ben
Ward, Zack Weinberg, Estelle Weyl, Boris Zbarsky, and all the rest of the
www-style
community.
References
Normative references
[CSS21]
Bert Bos; et al.
Cascading Style
Sheets Level 2 Revision 1 (CSS 2.1) Specification.
7 June
2011. W3C Recommendation. URL:
[CSS3-ANIMATIONS]
Dean Jackson; et al.
CSS
Animations.
19 February 2013. W3C Working Draft. (Work in
progress.) URL:
[CSS3-FONTS]
John Daggett.
CSS Fonts
Module Level 3.
12 February 2013. W3C Working Draft. (Work in
progress.) URL:
[MEDIAQ]
Florian Rivoal.
Media
Queries.
19 June 2012. W3C Recommendation. URL:
[RFC2119]
S. Bradner.
Key
words for use in RFCs to Indicate Requirement Levels.
Internet
RFC 2119. URL:
Other references
[CSS1]
Håkon Wium Lie; Bert Bos.
Cascading Style
Sheets (CSS1) Level 1 Specification.
11 April 2008. W3C
Recommendation. URL:
[CSS3-TRANSITIONS]
Dean Jackson; et al.
CSS
Transitions.
12 February 2013. W3C Working Draft. (Work in
progress.) URL:
Index
conditional group rules,
2.
conformance
authoring tool,
8.2.
processor,
8.2.
style sheet,
8.2.
CSS
7.6.
CSSConditionRule
7.3.
CSSGroupingRule
7.2.
CSSMediaRule
7.4.
CSSSupportsRule
7.5.
general_enclosed,
6.
group rule body,
3.
group_rule_body,
3.
media,
5.
@media
’ rule,
5.
nested_statement,
3.
OR,
6.
support,
6.1.
supports_condition,
6.
supports_condition_in_parens,
6.
supports_conjunction,
6.
supports_declaration_condition,
6.
supports_disjunction,
6.
supports_negation,
6.
@supports
’ rule,
6.
supports_rule,
6.
SUPPORTS_SYM,
6.