CSS3 Values and Units

Abstract

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 Dependencies on other modules
• 2 Introduction
• 3 Value definitions
• 4 Syntax and terminology
  • 4.1 Integers
  • 4.2 Numbers
  • 4.3 Numbers with unit identifiers
  • 4.4 Identifiers
  • 4.5 Strings
  • 4.6 Functional notation
  • 4.7 Cycle function
• 5 Keywords
• 6 Generic data types
  • 6.1 <integer>
  • 6.2 <number>
  • 6.3 <length>
    • 6.3.1 Absolute length units: cm, mm. in, pt, pc
    • 6.3.2 Relative length units
      • 6.3.2.1. The ‘em’ unit
      • 6.3.2.2. The ‘ex’ unit
      • 6.3.2.3. The ‘px’ unit
      • 6.3.2.4. The ‘rem’ unit
      • 6.3.2.5. The ‘vw’ unit
      • 6.3.2.6. The ‘vh’ unit
      • 6.3.2.7. The ‘vm’ unit
    • 6.3.3 The ‘calc’ function
  • 6.4 <string>
  • 6.5 <color>
  • 6.6 <percentage>
  • 6.7 <angle>
  • 6.8 <time>
  • 6.9 <frequency>
  • 6.10 <attr>
  • 6.11 <url>
  • 6.12 <image>
  • 6.13 <fraction>
    • 6.13.1 The ‘fr’ unit
  • 6.14 <grid>
    • 6.14.1 The ‘gr’ unit
• 7 Specified, computed, used, and actual values
  • 7.1 Finding the specified value
  • 7.2 Finding the computed value
  • 7.3 Finding the used value
  • 7.4 Finding the actual value
• Acknowledgments
• References
  • Normative references
  • Other references
• Index

1 Dependencies on other modules

This CSS3 module depends on the following other CSS3 modules:

• Cascading and Inheritance [CSS3CASCADE], which describes how to find the resulting value from several competing values. Also, it describes how to replace the "inherit" keyword with the inherited value.
• Syntax [CSS3SYN], which describes the syntax of values at the lexical level.
• Color [CSS3COLOR], which defines additional values used for some properties that control color of text, borders, etc
• Fonts [CSS3FONT], which describes additional values used for font-related properties

2 Introduction

By setting property values on elements in a document, style sheets express the appearance of the document. In order to express rich designs, a wide range of values and associated units are necessary. This specifiction describes the various types of values and units that can be used in CSS style sheets.

3 Value definitions

Each CSS property has a value definition field in the property description. The value definition describes what types of values the property accepts. The syntax used in the value definitions field is defined in [CSS3SYN].

The value definition fields 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.

4 Syntax and terminology

The generic data types described in the next sections use some common syntactic building blocks and terms that are described in this section.

4.1 Integers

An integer is one or more decimal digits "0" to "9". Integers may be preceded by "-" or "+" to indicate the sign.

orphans: 3

4.2 Numbers

A number is either an integer, or zero or more decimal digits followed by a dot (.) followed by one or more decimal digits. Numbers may be preceded by "-" or "+" to indicate the sign.

line-height: 1.2

4.3 Numbers with unit identifiers

A number with a unit identifier is a number immediately followed by a unit identifier.

border-left: -1.2em

4.4 Identifiers

An identifer is an seqence of characters. Identifiers cannot start with a digit, and there are other restrictions [CSS3SYN] on what characters an identifier can contain. Identifiers must not be quoted.

pitch-range: inherit;
counter-increment: header;

4.5 Strings

A string is a sequence of characters enclosed by double quotes or single quotes. 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\'';

A string cannot directly contain 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). See the ‘content’ property for an example.

It is possible to break strings over several lines, for esthetic 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"] {/*...*/}

4.6 Functional notation

Some values use a functional notation to type values and to and lump values together. The syntax starts with the name of the function followed by a left parenthesis followed by optional whitespace followed by the argument(s) to the functions 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 );

Some properties accept space- or comma-separated lists of values. A value that is composed of several values with spaces or commas between them, is called a compound value. A value that is not a compound value is a simple value.

4.7 Cycle function

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> [, <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 C[n] returned by the n-th argument to cycle(). For the earliest C[n] such that C[n] == I, the value returned by cycle is C[n+1]. However, if this C[n] is the last value, or if there are no C[n] that equal I, the computed value of the first value is returned instead.

  em { font-style: cycle(italic, normal); }

  ul { list-style-type: disk; }
  li > ul { list-style-type: cycle(disk, circle, square, box); }

Nested cycle() values are not allowed.

5 Keywords

In the value definition fields, keywords appear literally. Keywords are identifiers.

Value: collapse | separate

table { border-collapse: separate }

All CSS3 properties accept the keyword values ‘inherit’ and ‘initial’, as described in [CSS3CASCADE].

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).

6 Generic data types

6.1 <integer>

Integer values are denoted by <integer> in the value definitions. Properties may restrict the integer value to some range. If the value is outside the allowed range, the declaration is ignored.

6.2 <number>

Number values are denoted by <number> in the value definitions. Properties may restrict the number value to some range. If the value is outside the allowed range, the declaration is ignored.

6.3 <length>

Lengths are denoted by <length> in the property definitions. Lengths refer to horizontal or vertical measurements. Lengths are numbers with a unit identifier. After the ‘0’ length, the unit identifier is optional.

Some properties allow negative length values, but this may complicate the formatting and there may be implementation-specific limits. If a negative length value is allowed but cannot be supported, it should be converted to the nearest value that can be supported.

There are several types of length units. Relative length units specify a length relative to other lengths, while absolute length units express fixed lengths.

6.3.1 Absolute length units: cm, mm. in, pt, pc

Absolute length units are useful when the physical properties of the output medium are known. The absolute units are:

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 */

In cases where the specified length cannot be supported, user agents must approximate it in the actual value.

6.3.2 Relative length units

Relative units are:

6.3.2.1. The ‘em’ unit

The em unit is equal to the computed value of the ‘font-size’ property of the element on which it is used. The exception is when "em" occurs in the value of the ‘font-size’ property itself, in which case it refers to the font size of the parent element.

When specified on the ‘font-size’ property of the root element, the ‘em’ units refer to the property's initial value.

The rule:

h1 { line-height: 1.2em }

means that the line height of h1 elements will be 20% greater than the font size of the elements. 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.

h1 { margin: 0.5em }      /* em */
h1 { margin: 1ex }        /* ex */
p  { font-size: 12px }    /* px */

6.3.2.2. The ‘ex’ unit

The ex unit is defined by 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". Should we say that ex is 0.5em if no better value exists?

When specified on the ‘font-size’ property of the root element, the ‘ex’ units refer to the property's initial value.

6.3.2.3. The ‘px’ unit

The pixel unit, px is relative to the resolution of the viewing device. For example, the viewing device can be a computer display or a printer. Normally, the pixel unit refers to physical pixels of the viewing device. However, if the pixel density of the output device is very different from that of a typical computer display, the user agent should rescale pixel values. It is recommended that the reference pixel be 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 0.26mm (1/96 inch). When printed on a laser printer, meant for reading at a little less than arm's length (55 cm, 21 inches), 1px is about 0.21mm. On a 300 dots-per-inch (dpi) printer, that may be rounded up to 3 dots (0.25 mm); on a 600 dpi printer, it can be rounded to 5 dots.

The two images below illustrate the effect of viewing distance on the size of a pixel and the effect of a device's resolution. In the first image, a reading distance of 71cm (28 inch) results in a px of 0.26mm, while a reading distance of 3.5m (12 feet) requires a px of 1.3mm.

In the second image, an area of 1px by 1px is covered by a single dot in a low-resolution device (a computer screen), while the same area is covered by 16 dots in a higher resolution device (such as a 400 dpi laser printer).

6.3.2.4. The ‘rem’ unit

The rem unit ("root em") is relative 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.

6.3.2.5. The ‘vw’ unit

The vw unit is relative to the viewport's width. The viewport's width is equal to 100 ‘vw’ units.

h1 { font-size: 8vw }

If the width of the viewport is 200mm, the font size of h1 elements will be 16mm ((8×200)/100). When the width of the viewport is changed (for example, when the browser window is enlarged), lengths specifed in the ‘vh’ unit is scaled propertionally.

6.3.2.6. The ‘vh’ unit

The vh unit is relative to the viewport's height. The viewport's width is equal to 100 ‘vh’ units. When the height of the viewport is changed (for example, when the browser window is enlarged), lengths specifed in the ‘vh’ unit are scaled proportionally.

6.3.2.7. The ‘vm’ unit

The vm unit is relative to the viewport's height or width, whichever of the two is smaller. The minimum of the viewport's width/height is equivalent to 100 ‘vm’ units. When the height or width of the viewport is changed, lengths specified in the ‘vm’ unit are scaled proportionally.

6.3.3 The ‘calc’ function

The calc(<expression>) function can be used wherever length values are allowed, with the syntax given below.

section {
  float: left;
  margin: 1em; border: solid 1px;
  width: calc(100%/3 - 2*1em - 2*1px);
}

p {
  margin: calc(1rem - 2px) calc(1rem - 1px);
  border: solid transparent; border-width: 2px 1px;
}

The simple expression language of the ‘calc()’ function supports five arithmetic operators (+ and - have lowest precedence, *, /, and ‘mod’ have highest precedence) and parentheses.

A value is ignored if a division by zero or other mathemathical errors occur in the calculation.

p { 
 margin: 1em;
 margin: calc(1em-2px) calc(1em/0) 
}

Issue: At a later date new operators such as min/max, conditionals, new constants, division by length units etc. may be added.

The expression language is defined by ‘length-expression’ below:

  <length> := calc( <length-expression> ) | <atomic-length>

  <length-expression> := <length-additive-expression>

  <length-additive-expression> :=
     <length-multiplicative-expression> |
     <length-additive-expression> S+ '+' S+ <length-multiplicative-expression> |
     <length-additive-expression> S+ '-' S+ <length-multiplicative-expression>

  <length-multiplicative-expression>
     <length-term> |
     <length-multiplicative-expression> S* '*' S* <number-term> |
     <number-multiplicative-expression> S* '*' S* <length-term> |
     <length-multiplicative-expression> S* '/' S* <number-term> |
     <length-multiplicative-expression> S+ 'mod' S+ <number-term>

  <length-term> := '(' <length-expression> ')' | <atomic-length>

  <number-additive-expression> :=
     <number-multiplicative-expression> |
     <number-additive-expression> S+ '+' S+ <number-multiplicative-expression> |
     <number-additive-expression> S+ '-' S+ <number-multiplicative-expression>

  <number-multiplicative-expression> :=
     <number-term> |
     <length-multiplicative-expression> S* '/' S* <length-term> |
     <length-multiplicative-expression> S+ 'mod' S+ <length-term>

  <number-term> := '(' <number-additive-expression> ')' | <number>

  <atomic-length> := <number><length-unit>

6.4 <string>

Strings are denoted by <string> in the value definitions.

6.5 <color>

This section is not normative. The CSS3 Color module [CSS3COLOR] defines the CSS3 color values.

Color values are denoted by <color> in the value definitions.

A color value can either be a keyword, a numerical specification in a functional notation, or a numerical RGB specification in a hexadecimal notation. The hexadecimal notation is special shorthand format that allows compact color descriptions.

em { color: #F00 }
span.issue { color: red }
* { color: hsl(120, 75%, 75%) } 

6.6 <percentage>

The format of a percentage value, denoted by <percentage>, is a <number> immediately followed by ‘%’.

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.

p { font-size: 10pt }
p { line-height: 120% }  /* 120% of 'font-size' */

Do we need a "non-negative percentage", e.g. for "font-size"?

6.7 <angle>

Angle values (denoted by <angle> in the text) are used with aural cascading style sheets.

Their format is a <number> immediately followed by an angle unit identifier.

Angle unit identifiers are:

• deg: degrees
• grad: grads
• rad: radians
• turn: turns

Angle values should be normalized to the range 0-360deg by the user agent. For example, -10deg and 350deg are equivalent.

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

6.8 <time>

Time values are denoted by <time> in the text. Their format is a <number> immediately followed by a time unit identifier.

Time unit identifiers are:

• ms: milliseconds
• s: seconds

Time values are always positive.

6.9 <frequency>

Frequency values (denoted by <frequency> in the text) are used with aural cascading style sheets.

Their format is a <number> immediately followed by a frequency unit identifier.

Frequency unit identifiers are:

• Hz: Hertz
• kHz: kilohertz

For example, 200Hz (or 200hz) is a bass sound, and 6kHz (or 6khz) is a treble sound.

Frequency values are always positive.

6.10 <attr>

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.

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 UA dependent but must be the same as the initial value of the ‘color’ property.

url

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 non-descript error condition.)

integer

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.

number

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):

      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’.

      <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);
      }

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) }.

6.11 <url>

URLs (Uniform Resource Locators, see [RFC1738] and [RFC1808]) provide the address of a resource on the Web. An alternative and more general term is URIs (Uniform Resource Identifiers, see [!URI]). This specification uses the term URI.

For historical reasons, the name of the URI function is "url". The URI function takes one URI as the argument. The URI may be quoted with single quote (') or double quote (") characters. If quoted, the two quote characters must be the same.

body { background: url("http://www.example.com/pinkish.gif") }

li { list-style: url(http://www.example.com/redball.png) disc }

Parentheses, commas, whitespace characters, single quotes (‘) and double quotes (") appearing in a URI must be escaped with a backslash: ’\(‘, ’\)‘, ’\,‘.

Depending on the type of URI, it might also be possible to write the above characters as URI-escapes (where "(" = %28, ")" = %29, etc.) as described in [!URI].

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 [RFC1808]) are resolved to full URIs using a base URI. RFC 1808, 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.

body { background: url("yellow") }

http://www.example.org/style/basic.css

http://www.example.org/style/yellow

User agents may vary in how they handle URIs that designate unavailable or inapplicable resources.

URLs inside functional notation where URL is expected should be able to take either url() or bare strings.

6.12 <image>

TBD.

6.13 <fraction>

Some properties accept a series of length values that, in sum, should add up to a certain length. To take up any remaining space, fractions can be used.

6.13.1 The ‘fr’ unit

The fr unit is used to distribute any remaining space in a series of length values. If multiple fractions are specified, they take up space proportionally to their numeric value.

border-parts: 10px 1fr 10px;
border-parts: 10px 1fr 10px 1fr 10px;
border-parts: 10px 2fr 10px 2fr 10px;

The ‘fr’ unit can only be used in combination with regular length units.

6.14 <grid>

A grid is a set of invisible vertical and horizontal lines that can be used to align content. In CSS3, a grid lines can be established implicitly or explicitly [CSS3COL] [CSS3GRID]. In any case, the distance between grid lines can be referred to by the ‘gr’ unit.

6.14.1 The ‘gr’ unit

The gr unit is used to position elements in relation to grid lines.

img { 
  float: top left multicol; 
  float-offset: 2gr; 
  width: 1gr }
}

Grid lines can be laid out in uneven patterns. Therefore, the ‘gr’ unit is not linear.

7 Specified, computed, used, and actual values

The final value of a CSS3 property for a given element is the result of a three-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, therby yielding the computed value. Finally, the computed value is transformed to the actual value in the layout process.

7.1 Finding the specified value

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

7.2 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.

7.3 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.

7.4 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.

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.

References

Normative references

[CSS21]

Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 23 April 2009. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2009/CR-CSS2-20090423/

[CSS3CASCADE]

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

[CSS3COL]

Håkon Wium Lie. CSS3 module: Multi-column layout. 6 June 2007. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2007/WD-css3-multicol-20070606

[CSS3COLOR]

Chris Lilley; Tantek Çelik; L. David Baron. CSS Color Module Level 3. 21 July 2008. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2008/WD-css3-color-20080721

[CSS3FONT]

Michel Suignard; Chris Lilley. CSS3 module: Fonts. 2 August 2002. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2002/WD-css3-fonts-20020802

[CSS3GRID]

Markus Mielke; Alex Mogilevsky. CSS Grid Positioning Module Level 3. 5 September 2007. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2007/WD-css3-grid-20070905

[CSS3SYN]

L. David Baron. CSS3 module: Syntax. 13 August 2003. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2003/WD-css3-syntax-20030813

Other references

[CSS3NAMESPACE]

Anne van Kesteren; Elika J. Etemad. CSS Namespaces Module. 23 May 2008. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2008/CR-css3-namespace-20080523

[RFC1738]

T. Berners-Lee; L. Masinter; M. McCahill. Uniform Resource Locators (URL). December 1994. Internet RFC 1738. URL: http://www.ietf.org/rfc/rfc1738.txt

[RFC1808]

R. Fielding. Relative Uniform Resource Locators. June 1995. Internet RFC 1808. (Obsoleted by RFC 3986) URL: http://www.ietf.org/rfc/rfc1808.txt

Index

• absolute length units, 6.3
• actual value, 7.4
• <angle>
  • definition of, 6.7
• calc(<expression>), 6.3.3
• color, 6.5, 6.5
• compound value, 4.6
• computed value, 7.2
• deg (degrees), 6.7
• em (unit), 6.3.2.1.
• ex (unit), 6.3.2.2.
• <frequency>
  • definition of, 6.9
• fr (unit), 6.13.1
• functional notation, 4.6
• grad (grads), 6.7
• gr (unit), 6.14.1
• Hz (Hertz), 6.9
• identifer, 4.4
• <integer>, 6.1
• integer is one or more decimal digits "0" to "9". Integers may be preceded by "-" or "+" to indicate the sign. , 4.1
• kHz (kilohertz), 6.9
• <length>, 6.3
• ms (milliseconds), 6.8
• newline, 4.5
• <number>, 6.2, 6.6, 6.7, 6.8, 6.9, 4.2
• number with a unit identifier is a number immediately followed by a unit identifier. , 4.3
• <percentage>
  • definition of, 6.6
• pixel unit, 6.3.2.3.
• px (pixel), 6.3.2.3.
• rad (radians), 6.7
• reference pixel, 6.3.2.3.
• Relative length units, 6.3
• rem (unit), 6.3.2.4.
• Resource Identifier (URI), 6.11
• root em, 6.3.2.4.
• simple value, 4.6
• specified value, 7.1
• s (seconds), 6.8
• <string>, 6.4, 4.5
• <time>
  • definition of, 6.8
• turn, 6.7
• Uniform Resource Locator (URL), 6.11
• URI (Uniform Resource Identifier), 6.11
• URL (Uniform Resource Locator), 6.11
• used value, 7.3
• vh (unit), 6.3.2.6.
• vm (unit), 6.3.2.7.
• vw (unit), 6.3.2.5.
• x-height, 6.3.2.2.