Showing that pixels must become larger if the viewing distance increases
This CSS3 module describes the various values and units that CSS properties accept. Also, it describes how values are computed from "specified" through "computed" and "used" into "actual" values. The main purpose of this module is to define common values and units in one specification which can be referred to by other modules. As such, it does not make sense to claim conformance with this module alone.
All features described in this specification that also exist in CSS 2.1 [[!CSS21]] are intended to be backwards compatible. In case of conflict between this draft and CSS 2.1 [[!CSS21]], CSS 2.1 probably represents the intention of the CSS WG better than this draft (other than on values and units that are new to CSS3).
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.
This module replaces and extends the data type definitions in [[!CSS21]] sections 1.4.2.1, 4.3, and A.2.
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.
Component value types are designated in several ways:
auto)
<length>, <percentage>, etc.).
<'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 the value 'inherit'.
<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.
Component values can be arranged into property values as follows:
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 ]]]
Every type, keyword, or bracketed group may be followed by one of the following modifiers:
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.
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''.
Below are some examples of properties with their corresponding value definition fields
| Property | Value definition field | Example value |
|---|---|---|
| 'orphans' | <integer> | ''3'' |
| 'text-align' | left | right | center | justify | ''center'' |
| 'padding-top' | <length> | <percentage> | ''5%'' |
| 'outline-color' | <color> | invert | ''#fefefe'' |
| 'text-decoration' | none | underline || overline || line-through || blink | ''overline underline'' |
| 'font-family' | <family-name># | ''"Gill Sans", Futura, sans-serif'' |
| 'border-width' | [ <length> | thick | medium | thin ]{1,4} | ''2px medium 4px'' |
| 'text-shadow' | [ inset? && [ <length>{2,4} && <color>? ] ]# | none | ''3px 3px rgba(50%, 50%, 50%, 50%), lemonchiffon 0 0 4px inset'' |
| 'voice-pitch' | <frequency> && absolute | [[x-low | low | medium | high | x-high] || [<frequency> | <semitones> | <percentage>]] | ''-2st x-low'' |
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.
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 }
As defined above, all properties accept the ''initial'' and ''inherit'' keywords, which represent value computations common to all CSS properties.
The ''inherit'' keyword is defined in [[!CSS21]].
The ''initial'' keyword represents the specified value that is designated as the property's initial value. [[CSS3CASCADE]]
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).
What should be the minimum required size for identifiers? (This depends on the encoding, which means a character may take up to 6 bytes in utf-8.)
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:
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.)
What should be the minimum required size for strings? (This depends on the encoding, which means a character may take up to 6 bytes in utf-8.)
A URL is a pointer to a resource and is a
specially-parsed
functional notation denoted by
<url>. It corresponds to the
URI token in the
grammar.
[[!CSS21]]
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 (where ( = %28,
) = %29, etc.) 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.
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>.
What should be the minimum required size for urls? (This depends on the encoding, which means a character may take up to 6 bytes in utf-8.)
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.
We need a minimum required range/precision for dimensions as well. What should it be?
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.
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.
The relative units are:
| unit | relative to |
|---|---|
| ''em'' | font size of the element |
| ''ex'' | x-height of the element's font |
| ''ch'' | width of the "0" glyph in the element's font |
| ''rem'' | font size of the root element |
| ''vw'' | viewport's width |
| ''vh'' | viewport's height |
| ''vmin'' | minimum of the viewport's height and width |
Child elements do not inherit the relative values as specified for their parent; they inherit the computed values.
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).
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.
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.
Equal to the advance measure of the "0" (ZERO, U+0030) glyph found in the font used to render it.
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.
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.
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 }
Note that Paged Media defines how the initial containing block transforms across varying page widths. This also affects these 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:
| unit | definition |
|---|---|
| ''cm'' | centimeters |
| ''mm'' | millimeters |
| ''in'' | inches; 1in is equal to 2.54cm |
| ''px'' | pixels; 1px is equal to 1/96th of 1in |
| ''pt'' | points; 1pt is equal to 1/72nd of 1in |
| ''pc'' | picas; 1pc is equal to 12pt |
h1 { margin: 0.5in } /* inches */
h2 { line-height: 3cm } /* centimeters */
h3 { word-spacing: 4mm } /* millimeters */
h4 { font-size: 12pt } /* points */
h4 { font-size: 1pc } /* picas */
p { font-size: 12px } /* px */
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
Angle values are dimensions denoted by <angle>. The angle unit identifiers are:
For example, a right angle is '90deg' or '100grad' or '0.25turn' or approximately '1.570796326794897rad'.
Time values are dimensions denoted by <time>. The time unit identifiers are:
Properties may restrict the time value to some range. If the value is outside the allowed range, the declaration is invalid and must be ignored.
Frequency values are dimensions denoted by <frequency>. The frequency unit identifiers are:
For example, when representing sound pitches, 200Hz (or 200hz) is a bass sound, and 6kHz (or 6khz) is a treble sound.
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.
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.
background: url(http://www.example.org/image); color: rgb(100, 200, 50 ); content: counter(list-item) ". "; width: calc(50% - 2em);
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.
section {
float: left;
margin: 1em; border: solid 1px;
width: calc(100%/3 - 2*1em - 2*1px);
}
p {
margin: calc(1rem - 2px) calc(1rem - 1px);
}
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):
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.
The new syntax for the ''attr()'' expression is:
'attr(' wqname [ ',' <type> [ ',' <value> ]? ]? ')'
where wqname is a CSS qualified name [[!CSSNAMESPACE]] and whose syntax is defined as:
wqname : [ [ namespace_prefix? | '*' ] '|' ident ] | ident ;
The wqname argument 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 but must be present if the third argument is present) 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:
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.
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.