CSS3 Values and Units

Editor's Draft 10 August 2011

This version:
Latest version:
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Håkon Wium Lie, Opera Software <howcome@opera.com>
Chris Lilley, W3C <chris@w3.org>


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.

Status of this document

This is a public copy of the editors' draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don't cite this document other than as work in progress.

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 comment…

This document was produced by the CSS Working Group (part of the Style Activity).

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.

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).

This is a draft of a module of CSS level 3. It will probably be bundled with some other modules before it becomes a W3C Recommendation.

Table of contents

1. Introduction

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., <border-width>) are described in the corresponding modules.

1.1. Module Interactions

This module replaces and extends the data type definitions in [CSS21] sections, 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:

  1. keyword values (such as ‘auto’, ‘disc’, etc.), which appear literally, without quotes (e.g. auto)
  2. basic data types, which appear between ‘<’ and ‘>’ (e.g., <length>, <percentage>, etc.).
  3. 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 the value ‘inherit’.
  4. 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>. 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:

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:

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

Property Value definition field Example value
orphans<integer> 3
text-alignleft | right | center | justify center
padding-top<length> | <percentage> 5%
outline-color<color> | invert #fefefe
text-decorationnone | 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
<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. The ‘initial’ and ‘inherit’ keywords

As defined above, all properties accept the ‘initial’ and ‘inherit’ keywords, which represent value concepts common to all CSS properties.

The inherit keyword is defined in [CSS21].

The initial keyword represents the value that is designated as the property's initial value. [CSS3CASCADE]

Should these keywords affect the specified or computed value? See various issues.

Would it be useful to have a ‘default’ value, defined to be equivalent to inherit for properties that are inherited by default and equivalent to initial for properties that are not inherited by default? This might be easier for authors to use than initial and inherit since it wouldn't require thinking about whether a property is inherited by default or not (which isn't obvious for some properties, such as text-decoration and visibility).

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).

3.3. Quoted Strings: the ‘<string>’ type

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.)

3.4. Resource Locators: the ‘<url>’ type

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 [[RFC3986]]) 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:


The background of the source document's <body> will be tiled with whatever image is described by the resource designated by the URL:


The same image will be used regardless of the URL of the source document containing the <body<.

4. Numeric Data Types

4.1. Integers: the ‘<integer>’ type

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.

4.2. Numbers: the ‘<number>’ type

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.

4.3. Percentages: the ‘<percentage>’ type

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.

5. Distance Units: the ‘<length>’ type

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.

The relative units are:

Informative Summary of Relative Units
unit relative to
emfont size of the element
exx-height of the element's font
chwidth of the "0" glyph in the element's font
remfont size of the root element
vwviewport's width
vhviewport's height
vmminimum 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.

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-relative lengths: the ‘vw’, ‘vh’, ‘vm’ units

The viewport-relative lengths are relative to the size of the initial containing block. When the height or width of the viewport is changed, they are scaled proportionally.

vw unit
Equal to 1/100th 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 1/100th of the height of the initial containing block.
vm unit
Equal to the smaller of ‘vw’ or ‘vh’.

Do we need this now that we have the min() function?

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:

unit definition
ininches; 1in is equal to 2.54cm
pxpixels; 1px is equal to 1/96th of 1in
ptpoints; 1pt is equal to 1/72nd of 1in
pcpicas; 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.

This diagram illustrates how the definition of a pixel
    depends on the users distance from the viewing surface (paper or screen).
    The image depicts the user looking at two planes, one 28 inches (71 cm)
    from the user, the second 140 inches (3.5 m) from the user. An expanding
    cone is projected from the user's eye onto each plane. Where the cone
    strikes the first plane, the projected pixel is 0.26 mm high. Where the
    cone strikes the second plane, the projected pixel is 1.4 mm high.

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).

This diagram illustrates the relationship between the
    reference pixel and device pixels (called "dots" below). The
    image depicts a high resolution (large dot density) laser printer output
    on the left and a low resolution monitor screen on the right. For the
    laser printer, one square reference pixel is implemented by 16 dots. For
    the monitor screen, one square reference pixel is implemented by a single

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

6. Other Units

6.1. Angles: the ‘<angle>’ type and ‘deg’, ‘grad’, ‘rad’, ‘turn’ units

Angle values are dimensions denoted by <angle>. The angle unit identifiers are:

Degrees. There are 360 degrees in a full circle.
Gradians. There are 400 gradians in a full circle.
Radians. There are 2π radians in a full circle. Who is going to use this anyway?
Turns. There is 1 turn in a full circle.

For example, a right angle is ‘90deg’ or ‘100grad’ or ‘0.25turn’ or approximately ‘1.570796326794897rad’.

6.2. Times: the ‘<time>’ type and ‘s’, ‘ms’ units

Time values are dimensions denoted by <time>. The time unit identifiers are:

Milliseconds. There are 1000 milliseconds in a second.

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.

6.3. Frequencies: the ‘<frequency>’ type and ‘Hz’, ‘kHz’ units

Frequency values are dimensions denoted by <frequency>. The frequency unit identifiers are:

Hertz. It represents the number of occurrences per second.
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.

7. Data Types Defined Elsewhere

Some data types are defined in their own modules. The two common ones are <color> and <image>.

7.1. Colors: the ‘<color>’ type

The <color> data type is defined in [CSS21] and extended in [CSS3COLOR]. UAs that support CSS Color Level 3 must interpret <color> as defined therein.

7.2. Images: the ‘<image>’ type

The <image> data type is defined herein as equivalent to <url>. It is extended in [[!CSS3IMAGES]]: UAs that support CSS Image Values Level 3 must interpret <image> as defined therein.

8. Functional Notations

Some values use a functional notation to type values and to and 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 more than one argument, 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);

8.1. Calculations: ‘calc()’, ‘min()’ and ‘max()

The calc(), min(), and max() functions 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);
p { font-size: min(10px, 3em) }
blockquote { font-size: max(30px, 3em) }
.box { width: min(10% + 20px, 300px) }

The expression language of these functions is described by the grammar and prose below.

S       : calc | min | max;
calc    : "calc(" S* sum ")" S*;
min     : "min(" S* sum [ "," S* sum ]* ")" S*;
max     : "max(" S* sum [ "," S* sum ]* ")" S*;
sum     : product [ [ "+" | "-" ] S* product ]*;
product : unit [ [ "*" | "/" | "mod" ] S* unit ]*;
unit    : ["+"|"-"]? [ NUMBER S* | DIMENSION S* | PERCENTAGE S* |
          min | max | "(" S* sum ")" S* ];

The context of the expression imposes a target type, which is one of length, frequency, angle, time, or number. NUMBER tokens are of type number. DIMENSION tokens have types of their units (‘cm’ is length, ‘deg’ is angle etc.); any DIMENSION whose type does not match the target type is not allowed and must be a parse error. If percentages are accepted in that context and convertible to the target type, a PERCENTAGE token in the expression has the target type; otherwise percentages are not allowed and must be a parse error.

To make expressions simpler, operators have restrictions on the types they accept. At each operator, the types of the left and right side have to be checked for these restrictions. If compatible, they return roughly as follows (the following ignores precedence rules on the operators for simplicity):

  1. At ",", "+", "-":
    check: both sides have the same type
    return: that type
  2. At "*":
    check: at least one side is "number"
    return: the type of the other side
  3. At "/":
    check: right side is "number"
    return: the type of the left side

Division by zero is a parse error.

The value resulting from an expression must be clamped to the range allowed in the target context.

These two are equivalentequivalent to ‘width: 0px’ since widths smaller than 0px are not allowed.
width: calc(5px - 10px);
width: 0px;

Given the complexities of ‘width’ and ‘height’ on table cells and table elements, calc() expressions for ‘width’ and ‘height’ 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.

8.2. Cycling Values: ‘cycle()

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.

  /* 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()’, ‘calc()’, ‘min()’, or ‘max()’ notations. Declarations containing such constructs are invalid and must be ignored.

8.3. Attribute References: ‘attr()

Describe the feature fully here, not just a delta from CSS 21.

When attr is set on a pseudo-element, it should apply to the originating element

In CSS2.1 [CSS21], the ‘attr()’ expression always returns a string. In CSS3, the ‘attr()’ expression can return many different types. The new syntax for the attr() expression is:

'attr(' ident [ ',' <type> [ ',' <value> ]? ]? ')'

The first argument represents the attribute name. The value of the attribute with that name on the element whose computed values are being computed is used as the value of the expression, according to the rules given below.

The first argument accepts an optional namespace prefix to identify the namespace of the attribute. The namespace prefix and the attribute name is separated by ‘|’, with no whitespace before or after the separator [CSS3NAMESPACE].

The second argument (which is optional but must be present if the third argument is present) is a <type> and tells the UA how to interpret the attribute value. It may be one of the values from the list below.

The third argument (which is optional) is a CSS value which must be valid where the attr() expression is placed. If it is not valid, then the whole attr() expression is invalid.

If the attribute named by the first argument is missing, cannot be parsed, or is invalid for the property, then the value returned by attr() will be the third argument, or, if the third argument is absent, will be the value given as the default for the relevant type in the list below.

The attribute value will be interpreted as the contents of a CSS string. The default is the empty string.
The attribute value will be interpreted as a CSS <color> value. The default is UA dependent but must be the same as the initial value of the ‘color’ property.
The attribute value will be interpreted as the URI part 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.)
The attribute value will be interpreted as a CSS integer. The default is 0. 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. 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. 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).
em, ex, px, gr, rem, vw, vh, vm, mm, cm, in, pt, pc, deg, grad, rad, ms, s, Hz, kHz, %
The attribute value will be interpreted as a float, with the given type suffixed as a unit. The default is 0 in the relevant units.

Should there also be a "keyword" type to, e.g., support ‘float: attr(align)

If the <type> is missing, ‘string’ is implied.

Ideally, it shouldn't be necessary to specify the type if it is obvious. For example, this should be valid: "background-image: attr(href);". This could be described as: If the property only accepts one type of value (aside from ‘inherit’ and ‘initial’), that type is implied.

The attr() form is only valid if the type given (or implied, if it is missing) is valid for the property. For example, all of the following are invalid and would cause a parse-time error (and thus cause the relevant declaration, in this case all of them, to be ignored):

Illegal Examples:

      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 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.

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’.


        <wood length="12"/>
        <wood length="5"/>
        <metal length="19"/>
        <wood length="4"/>

      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);

The attr() expression cannot currently fall back onto another attribute. Future versions of CSS may extend attr() in this direction.

Should ‘attr()’ be allowed on any property, in any source language? For example, do we expect UAs to honor this rule for HTML documents?: P[COLOR] { color: attr(COLOR, color) }.

9. Specified, computed, used, and actual values

The final value of a CSS3 property for a given element is the result of a four-step calculation. First, cascading and inheritance yields the specified value [CSS3CASCADE]. Second, relative values are computed into absolute values as far as possible without formatting the document, thereby yielding the computed value. The computed value is transformed into the used value in the formatting process. Finally, the computed value is transformed to the actual value based on constraints in the user agent.

9.1. Finding the specified value

The specified value is the output of the cascading and inheritance process [CSS3CASCADE].

9.2. Finding the computed value

Specified values may be absolute (i.e., they are not specified relative to another value, as in ‘red’ or ‘2mm’) or relative (i.e., they are specified relative to another value, as in ‘auto’, ‘2em’). For absolute values, no processing is needed to find the computed value.

For relative values, on the other hand, computation is necessary to find the computed values: percentages must be multiplied by a reference value (each property defines which value that is), values with relative units (em, ex, px) must be made absolute by multiplying with the appropriate font or pixel size, ‘auto’ values must be computed by the formulas given with each property, certain keywords (e.g., ‘smaller’, ‘bolder’) must be replaced according to their definitions. See example (f), (g) and (h) in the table below.

Also, relative URIs are computed into absolute URIs at this stage. The computed value of invalid and absolute URIs is the same as the specified value.

9.3. Finding the used value

Computed values are processed as far as possible without formatting the document. Some values, however, can only be determined when the document is being laid out. For example, if the width of an element is set to be a certain percentage of its containing block, the width cannot be determined until the width of the containing block has been determined. The used value is the result of taking the computed value and resolving any remaining dependencies into an absolute value.

9.4. Finding the actual value

A used value is in principle ready to be used, but a user agent may not be able to make use of the value in a given environment. For example, a user agent may only be able to render borders with integer pixel widths and may therefore have to approximate the computed width. Also, the font size of an element may need adjustment based on the availability of fonts or the value of the ‘font-size-adjust’ property. The actual value is the computed value after adjustments have been made.

By probing the actual values of elements, much can be learned about how the document is laid out. However, not all information is recorded in the actual values. For example, the actual value of the ‘page-break-after’ property does not reflect whether there is a page break or not after the element. Similarly, the actual value of ‘orphans’ does not reflect how many orphan lines there is in a certain element. See examples (j) and (k) in the table below.

Example Winning declaration Property Specified value Computed value Used value Actual value
a text-align: left text-align left left left left
b border-width: inherit border-top-width, border-right-width, border-bottom-width, border-left-width 4.2px 4.2px 4.2px 4px
c (no winning declaration) width auto (initial value) auto 120px 120px
d list-style-position: inherit list-style-position inside inside inside inside
e list-style-position: initial list-style-position outside (initial value) outside outside outside
f font-size: 1.2em font-size 1.2em 14.1px 14.1px 14px
g width: 80% width 80% 80% 354.2px 354px
h width: auto width auto auto 134px 134px
i height: auto height auto auto 176px 176px
j (no winning declaration) page-break-after auto (initial value) auto auto auto
k orphans: 3 orphans 3 3 3 3


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.


Normative references

Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-CSS2-20110607
Håkon Wium Lie. CSS3 module: Cascading and inheritance. 15 December 2005. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2005/WD-css3-cascade-20051215
Tantek Çelik; Chris Lilley; L. David Baron. CSS Color Module Level 3. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-css3-color-20110607

Other references

Håkon Wium Lie. CSS3 module: Cascading and inheritance. 15 December 2005. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2005/WD-css3-cascade-20051215
Elika J. Etemad; Anne van Kesteren. CSS Namespaces Module. 23 May 2008. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2008/CR-css3-namespace-20080523/
T. Berners-Lee; R. Fielding; L. Masinter. Uniform Resource Identifiers (URI): generic syntax. January 2005. Internet RFC 3986. URL: http://www.ietf.org/rfc/rfc3986.txt