This CSS3 module describes the common values and units that CSS
properties accept and the syntax used for describing them in CSS property
definitions.
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/.
Publication as a Working Draft 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.
The (archived) public
mailing list www-style@w3.org (see
instructions) is preferred
for discussion of this specification. When sending e-mail, please put the
text “css3-values” in the subject, preferably like this:
“[css3-values] …summary of
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This specification is a Last Call Working Draft. All
persons are encouraged to review this document and send comments
to the www-style
mailing list as described above. The deadline for
comments is 29 March 2012.
All features described in this specification that also exist in CSS 2.1
[CSS21] are intended
to be backwards compatible. If you notice a conflict between this draft
and CSS 2.1 [CSS21], please inform the editors!
The following features are at-risk and may be dropped during the CR
period: ‘calc()’,
‘cycle()’,
‘attr()’.
The value definition field of each CSS property can contain keywords,
data types (which appear between ‘<’
and ‘>’), and information on how they
can be combined. Generic data types (<length> being the most widely
used) that can be used by many properties are described in this
specification, while more specific data types (e.g.,
<spacing-limit>) are described in the corresponding
modules.
1.1. Module Interactions
This module replaces and extends the data type definitions in [CSS21] sections 1.4.2.1, 4.3, and A.2.
2. Value Definition Syntax
The syntax described here is used to define the set of valid values for
CSS properties. A property value can have one or more components.
2.1. Component value types
Component value types are designated in several ways:
keyword values (such as ‘auto’, ‘disc’,
etc.), which appear literally, without quotes (e.g. auto)
basic data types, which appear between ‘<’ and ‘>’ (e.g., <length>, <percentage>, etc.).
types that have the same range of values as a property bearing the
same name (e.g., <'border-width'><'background-attachment'>, etc.). In this case, the
type name is the property name (complete with quotes) between the
brackets. Such a type does not include CSS-wide keywords such as ‘inherit’.
non-terminals that do not share the same name as a property. In this
case, the non-terminal name appears between ‘<’ and ‘>’, as in <spacing-limit>.
Notice the distinction between <border-width> and
<'border-width'>: the latter is defined as the value
of the ‘border-width’ property,
the former requires an explicit expansion elsewhere. The definition of a
non-terminal is located near its first appearance in the specification.
Some property value definitions also include the slash (/) and/or the
comma (,) as literals. These represent their corresponding tokens.
All CSS properties also accept the keyword values ‘inherit’ and ‘initial’ as their property value, but for
readability these are not listed explicitly in the property value syntax
definitions. These keywords cannot be combined with other component values
in same declaration; such a declaration is invalid. For example,
‘background: url(corner.png) no-repeat,
inherit;’ is invalid.
2.2. Component value
combinators
Component values can be arranged into property values as follows:
Several juxtaposed words mean that all of them must occur, in the
given order.
A double ampersand (&&) separates two or more components, all of which
must occur, in any order.
A double bar (||) separates two or more options: one or more of them
must occur, in any order.
A bar (|) separates two or more alternatives: exactly one of them must
occur.
Brackets ([ ]) are for grouping.
Juxtaposition is stronger than the double ampersand, the double
ampersand is stronger than the double bar, and the double bar is stronger
than the bar. Thus, the following lines are equivalent:
a b | c || d && e f
[ a b ] | [ c || [ d && [ e f ]]]
2.3. Component value
multipliers
Every type, keyword, or bracketed group may be followed by one of the
following modifiers:
An asterisk (*) indicates that the preceding type, word, or group
occurs zero or more times.
A plus (+) indicates that the preceding type, word, or group occurs
one or more times.
A question mark (?) indicates that the preceding type, word, or group
is optional.
A pair of numbers in curly braces ({A,B})
indicates that the preceding type, word, or group occurs at least
A and at most B times.
A hash mark (#) indicates that the preceding type, word, or group
occurs one or more times, separated by comma tokens.
For repeated component values (indicated by ‘*’, ‘+’, or
‘#’), UAs must support at least 30
repetitions of the component. If a property value contains more than the
supported number of repetitions, the declaration must be ignored as if it
were invalid.
2.4. Component values
and white space
Component values are specified in terms of tokens, as described in Chapter 4
of [CSS21]. As the
grammar allows spaces between tokens in the components of the
value production, spaces may appear between tokens in
property values.
Note: In many cases, spaces will in fact be required
between tokens in order to distinguish them from each other. For example,
the value ‘1em2em’ would be parsed as a
single DIMEN token with the number ‘1’ and the identifier ‘em2em’, which is an invalid unit. In this case, a
space would be required before the ‘2’
to get this parsed as the two lengths ‘1em’ and ‘2em’.
2.5. Property value
examples
Below are some examples of properties with their corresponding value
definition fields
<frequency> && absolute |
[[x-low | low | medium | high | x-high] ||
[<frequency> | <semitones> | <percentage>]]
‘-2st x-low’
3. Textual Data Types
An identifier is a sequence of characters
conforming to the IDENT token in the grammar.
[CSS21] Identifiers
cannot be quoted; otherwise they would be interpreted as a string.
3.1. Pre-defined Keywords
In the value definition fields, keywords with a pre-defined meaning
appear literally. Keywords are CSS identifiers and are interpreted
case-insensitively within the ASCII range (i.e., [a-z] and [A-Z] are
equivalent).
For example, here is the value definition for the ‘border-collapse’ property:
Value: collapse | separate
And here is an example of its use:
table { border-collapse: separate }
3.1.1. CSS-wide keywords:
‘initial’ and ‘inherit’
As defined above, all properties accept
the ‘initial’ and ‘inherit’ keywords, which represent value
computations common to all CSS properties.
The ‘initial’
keyword represents the specified value that is designated as the
property's initial value. [CSS3CASCADE]
3.2. User-defined Identifiers:
the ‘<identifier>’ type
Some properties accept arbitrary user-defined identifiers as a component
value. This generic data type is denoted by <identifier>, and represents
any valid CSS identifier that does not
otherwise appear as a pre-defined keyword in that property's value
definition. Such identifiers are fully case-sensitive, even in the ASCII
range (e.g. ‘example’ and ‘EXAMPLE’ are two different, unrelated user-defined
identifiers).
Strings are denoted by <string> and consist of a
sequence of characters delimited by double quotes or single quotes. They
correspond to the STRING token in the grammar.
[CSS21]
Double quotes cannot occur inside double quotes, unless escaped (as
"\"" or as "\22"). Analogously for single
quotes ('\'' or '\27').
content: "this is a 'string'.";
content: "this is a \"string\".";
content: 'this is a "string".';
content: 'this is a \'string\'.';
It is possible to break strings over several lines, for aesthetic or
other reasons, but in such a case the newline itself has to be escaped
with a backslash (\). The newline is subsequently removed from the string.
For instance, the following two selectors are exactly the same:
Example(s):
a[title="a not s\
o very long title"] {/*...*/}
a[title="a not so very long title"] {/*...*/}
Since a string cannot directly represent a newline, to include a newline
in a string, use the escape "\A". (Hexadecimal A is the line feed
character in Unicode (U+000A), but represents the generic notion of
"newline" in CSS.)
Below is an example of a URL being used as a background image:
body { background: url("http://www.example.com/pinkish.gif") }
The same example can be written without quotes:
body { background: url(http://www.example.com/pinkish.gif) }
Note that in some CSS syntactic contexts (as defined by that
context), a URL can be represented as a <string> rather than by <URL>. An example of this is the
‘@import’ rule.
Parentheses, whitespace characters, single quotes (') and double
quotes (") appearing in a URL must be escaped with a backslash so that the
resulting value is a valid URL token, e.g.
‘url(open\(parens)’, ‘url(close\)parens)’. Depending on the type of URL,
it might also be possible to write these characters as URI-escapes (e.g.
‘url(open%28parens)’ or ‘url(close%29parens)’) as described in [URI]. Alternatively a URL
containing such characters may be represented as a quoted string within the ‘url()’ notation.
In order to create modular style sheets that are not dependent on the
absolute location of a resource, authors should use relative URIs.
Relative URIs (as defined in [URI]) are resolved to full URIs using a
base URI. RFC 3986, section 3, defines the normative algorithm
for this process. For CSS style sheets, the base URI is that of the style
sheet, not that of the source document.
When a <url> appears in the
computed value of a property, it is resolved to an absolute URL, as
described in the preceding paragraph.
For example, suppose the following rule:
body { background: url("tile.png") }
is located in a style sheet designated by the URL:
http://www.example.org/style/basic.css
The background of the source document's <body> will
be tiled with whatever image is described by the resource designated by
the URL:
http://www.example.org/style/tile.png
The same image will be used regardless of the URL of the source
document containing the <body>.
Integer values are denoted by <integer>. An integer is one or more decimal digits ‘0’ through ‘9’
and corresponds to a subset of the NUMBER token in the grammar.
Integers may be immediately preceded by ‘-’ or ‘+’ to
indicate the sign.
Properties may restrict the integer value to some range. If the value is
outside the allowed range, the declaration is invalid and must be ignored. For
unrestricted values, UAs must support at least up to ±230;
unsupported values must be clamped to the closest supported value.
Number values are denoted by <number>. A number is either an integer,
or zero or more decimal digits followed by a dot (.) followed by one or
more decimal digits. It corresponds to the NUMBER token in the grammar.
Like integers, numbers may also be immediately preceded by ‘-’ or ‘+’ to
indicate the sign.
Properties may restrict the number value to some range. If the value is
outside the allowed range, the declaration is invalid and must be ignored. For
unrestricted values, UAs must support at least up to ±230;
unsupported values must be clamped to the closest supported value.
A percentage value is denoted by <percentage>, consists of a
<number> immediately followed by
a percent sign ‘%’. It corresponds to
the PERCENTAGE token in the grammar.
Percentage values are always relative to another value, for example a
length. Each property that allows percentages also defines the value to
which the percentage refers. The value may be that of another property for
the same element, a property for an ancestor element, or a value of the
formatting context (e.g., the width of a containing block). When a
percentage value is set for a property of the root element and the
percentage is defined as referring to the inherited value of some
property, the resultant value is the percentage times the initial
value of that property.
Properties may restrict the percentage value to some range. If the value
is outside the allowed range, the declaration is invalid and must be ignored. For
unrestricted values, UAs must support at least up to ±230%;
unsupported values must be clamped to the closest supported value.
Lengths refer to distance measurements and are denoted by <length> in the property
definitions. A length is a dimension. A
zero length may be represented instead as the <number> ‘0’. (In other words, for zero lengths the unit
identifier is optional.)
A dimension is a number immediately followed by a unit
identifier. It corresponds to the DIMENSION token in the grammar.
[CSS21] Like
keywords, unit identifiers are case-insensitive within the ASCII range.
Properties may restrict the length value to some range. If the value is
outside the allowed range, the declaration is invalid and must be ignored.
While some properties allow negative length values, this may complicate
the formatting and there may be implementation-specific limits. If a
negative length value is allowed but cannot be supported, it must be
converted to the nearest value that can be supported.
In cases where the used length cannot be
supported, user agents must approximate it in the
actual value.
There are two types of length units: relative and absolute.
5.1. Relative lengths
Relative length
units specify a length relative to another length. Style sheets that
use relative units can more easily scale from one output environment to
another.
Child elements do not inherit the relative values as specified for their
parent; they inherit the computed values.
5.1.1. Font-relative
lengths: the ‘em’, ‘ex’, ‘ch’, ‘rem’ units
Aside from ‘rem’ (which refers to the font-size of the root
element), the font-relative lengths refer to the computed font metrics of
the element on which they are used. The exception is when they occur in
the value of the ‘font-size’
property itself, in which case they refer to the font metrics of the
parent element (or the font metrics corresponding to the initial values of
the ‘font’ property, if the
element has no parent).
em unit
Equal to the computed value of the ‘font-size’ property of the element on which
it is used.
The rule:
h1 { line-height: 1.2em }
means that the line height of h1 elements will be 20%
greater than the font size of h1 element. On the other
hand:
h1 { font-size: 1.2em }
means that the font size of h1 elements will be 20%
greater than the font size inherited by h1 elements.
ex unit
Equal to the font's x-height. The x-height is so called because it is
often equal to the height of the lowercase "x". However, an ‘ex’ is defined even for
fonts that do not contain an "x".
The x-height of a font can be found in different ways. Some fonts
contain reliable metrics for the x-height. If reliable font metrics are
not available, UAs may determine the x-height from the height of a
lowercase glyph. One possible heuristic is to look at how far the glyph
for the lowercase "o" extends below the baseline, and subtract that
value from the top of its bounding box. In the cases where it is
impossible or impractical to determine the x-height, a value of 0.5em
must be assumed.
ch unit
Equal to the advance measure of the "0" (ZERO, U+0030) glyph found in
the font used to render it.
rem unit
Equal to the computed value of ‘font-size’ on the root element.
When specified on the ‘font-size’ property of the root element, the
‘rem’ units
refer to the property's initial value.
5.1.2.
Viewport-percentage lengths: the ‘vw’, ‘vh’, ‘vmin’ units
The viewport-percentage lengths are relative to the size of the initial
containing block. When the height or width of the viewport is changed,
they are scaled accordingly.
vw unit
Equal to 1% of the width of the initial containing block.
In the example below, if the width of the viewport is 200mm, the font
size of h1 elements will be 16mm (i.e.
(8×200mm)/100).
h1 { font-size: 8vw }
vh unit
Equal to 1% of the height of the initial containing block.
Note that Paged Media defines how the initial containing
block transforms across varying page widths. This also affects these
units.
5.2. Absolute lengths:
the ‘cm’, ‘mm’, ‘in’,
‘pt’, ‘pc’, ‘px’ units
The absolute length units are fixed
in relation to each other and anchored to some physical measurement. They
are mainly useful when the output environment is known. The absolute units
consist of the physical units (in, cm, mm, pt, pc) and the px unit:
For a CSS device, these dimensions are either anchored (i) by relating
the physical units to their physical measurements, or (ii) by relating the
pixel unit to the reference pixel.
For print media and similar high-resolution devices, the anchor unit
should be one of the standard physical units (inches, centimeters, etc).
For lower-resolution devices, and devices with unusual viewing distances,
it is recommended instead that the anchor unit be the pixel unit. For such
devices it is recommended that the pixel unit refer to the whole number of
device pixels that best approximates the reference pixel.
Note that if the anchor unit is the pixel unit, the physical
units might not match their physical measurements. Alternatively if the
anchor unit is a physical unit, the pixel unit might not map to a whole
number of device pixels.
Note that this definition of the pixel unit and the physical
units differs from previous versions of CSS. In particular, in previous
versions of CSS the pixel unit and the physical units were not related by
a fixed ratio: the physical units were always tied to their physical
measurements while the pixel unit would vary to most closely match the
reference pixel. (This change was made because too much existing content
relies on the assumption of 96dpi, and breaking that assumption breaks the
content.)
The reference pixel is the visual angle of
one pixel on a device with a pixel density of 96dpi and a distance from
the reader of an arm's length. For a nominal arm's length of 28 inches,
the visual angle is therefore about 0.0213 degrees. For reading at arm's
length, 1px thus corresponds to about 0.26 mm (1/96 inch).
The image below illustrates the effect of viewing distance on the size
of a reference pixel: a reading distance of 71 cm (28 inches)
results in a reference pixel of 0.26 mm, while a reading distance of
3.5 m (12 feet) results in a reference pixel of 1.3 mm.
Showing that pixels must become larger if the viewing
distance increases
This second image illustrates the effect of a device's resolution on the
pixel unit: an area of 1px by 1px is covered by a single dot in a
low-resolution device (e.g. a typical computer display), while the same
area is covered by 16 dots in a higher resolution device (such as a
printer).
Showing that more device pixels (dots) are needed to
cover a 1px by 1px area on a high-resolution device than on a low-res one
Frequency values are dimensions denoted
by <frequency>. The frequency unit
identifiers are:
Hz
Hertz. It represents the number of occurrences per second.
kHz
KiloHertz. A kiloHertz is 1000 Hertz.
For example, when representing sound pitches, 200Hz (or 200hz) is a bass
sound, and 6kHz (or 6khz) is a treble sound.
6.4. Resolutions: the ‘dpi’, ‘dpcm’, and ‘dppx’ units
Resolution units are dimensions denoted
by <resolution>. The frequency unit
identifiers are:
dpi
dots per inch
dpcm
dots per centimeter
dppx
dots per ‘px’ unit
The ‘<resolution>’ unit represents the size of a
single "dot" in a graphical representation by indicating how many of these
dots fit in a CSS ‘in’,
‘cm’, or ‘px’. For uses, see e.g. the ‘resolution’ media query in [MEDIAQ] or the ‘image-resolution’ property defined in [CSS3-IMAGES].
Note that due to the 1:96 fixed ratio of CSS ‘in’ to CSS ‘px’,
‘1dppx’ is equivalent to ‘96dpi’. This corresponds to the default resolution
of images displayed in CSS: see ‘image-resolution’.
The following @media rule uses Media Queries [MEDIAQ] to assign some special
style rules to devices that use two or more device pixels per CSS
‘px’ unit:
@media (min-resolution: 2dppx) { ... }
7. Data Types Defined
Elsewhere
Some data types are defined in their own modules. The two common ones
are <color> and <image>.
The <color> data type is defined in
[CSS21] and extended in [CSS3COLOR].
UAs that support CSS Color Level 3 or its successor must interpret <color> as defined therein.
The <image> data type is
defined herein as equivalent to <url>. It is extended in [CSS3-IMAGES]: UAs that support
CSS Image Values Level 3 or its successor must interpret <image> as defined therein.
The <position> data type
is defined herein as equivalent to the <'background-position'>
syntax defined in [CSS21]. It specifies the position of
a object area (e.g. background image) inside a positioning area (e.g.
background positioning area). It is extended
in [CSS3BG]: UAs
that support CSS Backgrounds & Borders Level 3 or its successor must
interpret <position> as
defined therein.
8. Functional
Notations
Some values use a functional notation
to type values and to lump values together. The syntax starts with the
name of the function immediately followed by a left parenthesis followed
by optional whitespace followed by the argument(s) to the notation
followed by optional whitespace followed by a right parenthesis. If a
function takes a list of arguments, the arguments are separated by a comma
(‘,’) with optional whitespace before
and after the comma.
The calc() function allows mathematical expressions
with addition (‘+’), subtraction
(‘-’), multiplication (‘*’), and division (‘/’) to be used as component values. The ‘calc()’ expression
represents the result of the mathematical calculation it contains, using
standard operator precedence rules. It can be used wherever <length>, <frequency>, <angle>, <time>, or <number> values are allowed.
The following sets the ‘font-size’ so that exactly 40em fits within
the viewport, ensuring that roughly the same amount of text always fills
the screen no matter the screen size.
:root {
font-size: calc(100vw / 40);
}
If the rest of the design is specified using the ‘rem’ unit, the entire
layout will scale to match the viewport width.
The expression language of these functions is described by the grammar
and prose below.
math : calc S*;
calc : "calc(" S* sum S* ")";
sum : product [ S+ [ "+" | "-" ] S+ product ]*;
product : unit [ S* [ "*" | "/" ] S* unit ]*;
unit : ["+"|"-"]? [ NUMBER | DIMENSION | PERCENTAGE | "(" S* sum S* ")" ];
Note that the grammar requires spaces around binary
‘+’ and ‘-’ operators. The ‘*’ and ‘/’
operators do not require spaces.
Additionally, the following redefinition is made to the informative
grammar appearing in CSS
2.1 Appendix G:
term
: unary_operator?
[ NUMBER S* | PERCENTAGE S* | LENGTH S* | EMS S* | EXS S* | ANGLE S* |
TIME S* | FREQ S* ]
| STRING S* | IDENT S* | URI S* | hexcolor | function | math
;
A math expression has a resolved type, which
is one of ‘<length>’, ‘<frequency>’, ‘<angle>’, ‘<time>’, or ‘<number>’. The resolved type must be valid for where the
expression is placed; otherwise, the expression is invalid. The resolved type of the expression is
determined by the types of the values it contains. NUMBER tokens are of type ‘<number>’. A DIMENSION token's type is given by its
unit (‘cm’ is ‘<length>’, ‘deg’ is ‘<angle>’, etc.). If percentages are accepted in
the context in which the expression is placed, a PERCENTAGE token has the type of the
value that percentages are relative to; otherwise, a math expression
containing percentages is invalid.
Operators form sub-expressions, which gain types based on their
arguments. To make expressions simpler, operators have restrictions on the
types they accept. At each operator, the types of the left and right
argument are checked for these restrictions. If compatible, the type
resolves as described below (the following ignores precedence rules on the
operators for simplicity):
At ‘,’, ‘+’, or ‘-’,
check that both sides have the same type; resolve to that type.
At ‘*’, check that at least one
side is ‘<number>’; resolve to the
type of the other side
At ‘/’, check that the right side
is ‘<number>’; resolve to the type
of the left side.
If an operator does not pass the above checks, the expression is
invalid. Also, division by zero is invalid. This includes both dividing by
the literal number zero, as well as any numeric expression that evaluates
to zero (as purely-numeric expressions can be evaluated without any
additional information at parse time).
The value resulting from an expression must be clamped to the range
allowed in the target context.
Note this requires all contexts accepting ‘calc()’ to define their
allowable values as a closed (not open) interval.
These two are equivalent to ‘width:
0px’ since widths smaller than 0px are not allowed.
width: calc(5px - 10px);
width: 0px;
Given the complexities of width and height calculations on table cells
and table elements, math expressions involving percentages for widths and
heights on table columns, table column groups, table rows, table row
groups, and table cells in both auto and fixed layout tables MAY be
treated as if ‘auto’ had been
specified.
UAs must support ‘calc()’ expressions of at least 30 terms, where
each ‘number’,
‘dimension’,
or ‘percentage’ is a term. If a ‘calc()’ expression contains
more than the supported number of terms, it must be treated as if it were
invalid.
The cycle() expression allows descendant
elements to cycle over a list of values instead of inheriting the same
value. The syntax of the ‘cycle()’ expression is:
cycle( <value># )
where <value> is a CSS value that is valid where the
expression is placed. If any of the values inside are not valid, then the
entire ‘cycle()’ expression is invalid.
The value returned by ‘cycle()’ must be determined by comparing the
inherited value I (the computed value on the parent, or, for
the root, the initial value) to the computed values
Cn returned by the n-th argument to
‘cycle()’.
For the earliest Cn such that
Cn = I, the value returned by cycle is
Cn+1. However, if this Cn is
the last value, or if there are no Cn that equal
I, the computed value of the first value is returned instead.
Note that ‘cycle()’ explicitly looks at the computed value
of the parent, so it is useful even for non-inherited properties. This is
similar to the ‘inherit’ keyword, which
is useful even for non-inherited properties.
/* make em elements italic, but make them normal if they're inside
something that's italic */
em { font-style: cycle(italic, normal); }
/* cycle between markers for nested lists, so that the top level has
disk markers, but nested lists use circle, square, box, and then
(for the 5th list deep) repeat */
ul { list-style-type: disk; }
li > ul { list-style-type: cycle(disk, circle, square, box); }
The ‘cycle()’ notation is not allowed to be nested;
nor may it contain ‘attr()’ or ‘calc()’ notations. Declarations containing such
constructs are invalid.
The attr() function returns the value of an
attribute on the element for use as a value in a property. If used on a
pseudo-element, it returns the value of the attribute on the
pseudo-element's originating element.
In CSS2.1 [CSS21],
the ‘attr()’
expression always returns a string. In CSS3, the ‘attr()’ expression can
return many different types. The ‘attr()’ expression cannot return everything,
for example it cannot do counters, named strings, quotes, or values such
as ‘auto’, ‘nowrap’, or ‘baseline’. This is intentional, as the intent of
the ‘attr()’
expression is not to make it possible to describe a presentational
language's formatting using CSS, but to enable CSS to take semantic data
into account.
where wqname is a CSS qualified
name[CSS3NAMESPACE] that
represents an attribute name. The computed value of the ‘attr()’ expression is
the value of the attribute with that name on the element, according to the
rules given below.
The ‘<type>’ argument (which is
optional) is a keyword drawn from the list below that tells the UA how to
interpret the attribute value. If omitted, ‘string’ is implied. If the type is not valid for
where the ‘attr()’ expression is placed, the whole
‘attr()’
expression is invalid.
The ‘<value>’ argument (which is
optional) is a CSS value which must be valid where the ‘attr()’ expression is
placed. It represents a fallback value to be used if the named attribute
is missing, or its value cannot be parsed into the given type or is
invalid/out-of-range for the property. If the ‘<value>’ argument is not valid for the property
where the ‘attr()’ expression is placed, the whole
‘attr()’
expression is invalid. The fallback value must not be another ‘attr()’ expression; if
it is, the outer ‘attr()’ expression is invalid. If the fallback
‘<value>’ is absent, the default
value for the given type (from the list below) is implied.
Note that the default value need not be of the type given.
For instance, if the type required of the attribute by the author is
‘px’, the default could still be
‘5em’.
The ‘<type>’ keywords are:
‘string’
The attribute value will be interpreted as the contents of a CSS
‘<string>’. The default is the
empty string.
‘color’
The attribute value will be interpreted as a CSS ‘<color>’ value. The default is ‘currentColor’.
The attribute value will be interpreted as the argument of a ‘url()’ expression. The
default is a UA-dependent URI defined to point to a non-existent document
with a generic error condition. (i.e. it shouldn't be an FTP URI that
causes a DNS error, or an HTTP URI that results in a 404, it should be a
nondescript error condition.) Relative URLs must be made absolute
according to the rules of the document language as applied to URLs
originating from the element; they are not relative to the style sheet.
The attribute value will be interpreted as a CSS ‘<integer>’. The default is ‘0’, or else the property's minimum value if
‘0’ is not valid for the property. The
default should also be used if the property in question only accepts
integers within a certain range and the attribute is out of range.
The attribute value will be interpreted as a CSS ‘<number>’. The default is ‘0.0’, or else the property's minimum value if
‘0.0’ is not valid for the property.
The default should also be used if the property in question only accepts
numbers within a certain range and the attribute is out of range.
‘length’
‘angle’
‘time’
‘frequency’
The attribute value will be interpreted as a CSS ‘<length>’, ‘<angle>’, ‘<time>’ or ‘<frequency>’ (respectively), and the unit
identifier (if any) will appear in the attribute value. The default is
‘0’ in the relevant units, or else the
property's minimum value if ‘0’ in the
relevant units is not valid for the property. The default should also be
used if the property in question only accepts values within a certain
range (e.g. positive lengths or angles from 0 to 90deg) and the attribute
is out of range (e.g. a negative length or 180deg).
The attribute value will be interpreted as a CSS ‘<number>’, with the given type suffixed as a
unit. The default is ‘0’ in the
relevant units, or else the property's minimum value if ‘0’ in the relevant units is not valid for the
property.
This example shows the use of attr() to visually illustrate data in an
XML file:
<stock>
<wood length="12"/>
<wood length="5"/>
<metal length="19"/>
<wood length="4"/>
</stock>
stock::before {
display: block;
content: "To scale, the lengths of materials in stock are:";
}
stock > * {
display: block;
width: attr(length em); /* default 0 */
height: 1em;
border: solid thin;
margin: 0.5em;
}
wood {
background: orange url(wood.png);
}
metal {
background: silver url(metal.png);
}
/* this also uses a possible extension to the 'content' property
to handle replaced content and alternatives to unavailable,
corrupted or unsupported content */
img {
content: replaced attr(src url), attr(alt string, none);
height: attr(height px, auto);
width: attr(width px, auto);
}
All of the following examples are invalid and would cause a parse-time
error, and thus cause the relevant declaration—in this case all of
them—to be ignored:
content: attr(title color); /* 'content' doesn't accept colors */
content: attr(end-of-quote string, inherit) close-quote;
/* the 'inherit' value is not allowed there, since the result would be
'inherit close-quote', which is invalid. */
margin: attr(vertical length) attr(horizontal deg);
/* deg units are not valid at that point */
color: attr(color); /* 'color' doesn't accept strings */
The ‘attr()’ expression cannot currently fall back
onto another attribute. Future versions of CSS may extend ‘attr()’ in this
direction.
Acknowledgments
Comments and suggestions from Giovanni Campagna, Christoph Päper,
Keith Rarick, Alex Mogilevsky, Ian Hickson, David Baron, Edward Welbourne,
Boris Zbarsky, Björn Höhrmann and Michael Day improved this
module.
9. Conformance
9.1. 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.
9.2. Conformance
classes
Conformance to CSS Values and Units Level 3 is defined for three
conformance classes:
A style sheet is conformant to CSS Values and Units Level 3 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 CSS Values and Units Level 3 if, in addition
to interpreting the style sheet as defined by the appropriate
specifications, it supports all the features defined CSS Values and Units
Level 3 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 CSS Values and Units Level 3 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.
9.3. 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.
9.4. Experimental
implementations
To avoid clashes with future CSS features, the CSS2.1 specification
reserves a prefixed
syntax for proprietary and experimental extensions to CSS.
Prior to a specification reaching the Candidate Recommendation stage in
the W3C process, all implementations of a CSS feature are considered
experimental. The CSS Working Group recommends that implementations use a
vendor-prefixed syntax for such features, including those in W3C Working
Drafts. This avoids incompatibilities with future changes in the draft.
9.5. 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.
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.
