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

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

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

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.

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.

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.

Integers

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

orphans: 3

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

Numbers with unit identifiers

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

border-left: -1.2em

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;

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\'';

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"] {/*...*/}

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.

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> ]*)

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.

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).

Generic data types

<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.

<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.

<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.

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.

Relative length units

Relative units are:

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 */

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.

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).

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.

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.

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.

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.

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>

<string>

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

<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%) } 

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

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

      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 */

      <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);
      }

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

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

body { background: url("yellow") }

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

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

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

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