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).
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.
This CSS3 module depends on the following other CSS3 modules:
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 specification describes the various types of values and units that can be used in CSS style sheets.
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]].
Here are some sample properties with corresponding value definition fields:
| Property | Value definition field |
|---|---|
| min-width | <length> | <percentage> | inherit |
| outline-color | <color> | invert | inherit |
| orphans | <integer> | inherit |
| pitch | <frequency> | x-low | low | medium | high | x-high | inherit |
| string-set | [[ <identifier> <content-list>] [, <identifier> <content-list>]* ] | none |
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.
The generic data types described in the next sections use some common syntactic building blocks and terms that are described in this section.
An integer is one or more decimal digits "0" to "9". Integers may be preceded by "-" or "+" to indicate the sign.
orphans: 3
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
A number with a unit identifier is a number immediately followed by a unit identifier.
border-left: -1.2em
An identifier is an sequence 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;
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 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"] {/*...*/}
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.
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.
In the value definition fields, keywords appear literally. Keywords are identifiers.
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 }
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).
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 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.
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 are useful when the physical properties of the output medium are known. The absolute units are:
| unit | definition |
|---|---|
| cm | centimeters |
| mm | millimeters |
| in | inches; 1 inch is equal to 2.54 centimeters. |
| pt | points; 1pt is equal to 1/72 inch. |
| pc | picas; 1 pica is equal to 12 points. |
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 units are:
| unit | relative to |
|---|---|
| em | the font size of the element (or, to the parent element's font size if set on the 'font-size' property) |
| ex | the x-height of the element's font |
| px | viewing device |
| rem | the font size of the root element |
| vw | the viewport's width |
| vh | the viewport's height |
| vm | the viewport's height or width, whichever is smaller of the two |
| ch | The width of the "0" (ZERO, U+0030) glyph found in the font for the font size used to render. If the "0" glyph is not found in the font, the average character width may be used. How is the "average character width" found? |
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 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 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 ("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 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 specified in the 'vw' unit are scaled
proportionally.
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 specified in the 'vh' unit are scaled proportionally.
Do we need this now that we have the min() function?
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(), 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):
Division by zero is a parse error.
The value resulting from an expression must be clamped to the range allowed in the target context.
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 are treated as if 'auto' had been specified.
Strings are denoted by <string> in the value definitions.
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%) }
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.
Since child elements (generally) inherit the computed values of their parent, in the following example, the children of the P element will inherit a value of 12pt for 'line-height', not the percentage value (120%):
p { font-size: 10pt }
p { line-height: 120% } /* 120% of 'font-size' */
Do we need a "non-negative percentage", e.g. for "font-size"?
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:
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'.
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:
Time values are always positive.
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:
For example, 200Hz (or 200hz) is a bass sound, and 6kHz (or 6khz) is a treble sound.
Frequency values are always positive.
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.
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'.
Examples:
<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) }.
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") }
An example without quotes:
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.
For example, suppose the following rule:
body { background: url("yellow") }
is located in a style sheet designated by the URI:
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 URI
http://www.example.org/style/yellow
User agents may vary in how they handle URIs that designate unavailable or inapplicable resources.
TBD.
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.
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.
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.
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.
For example, "2gr" is not necessarily twice as long as "1gr".
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.
The specified value is the output of the cascading and inheritance process [[!CSS3CASCADE]].
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.
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.
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.