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).
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).
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:
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.)
Below is an example of a URL being used as a background image:
body { "www.example.com/pinkish.gif") }
The same example can be written without quotes:
body { ") 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 (e.g.
‘url(open%28parens)’ or ‘url(close%29parens)’) as described in [URI]. Alternatively a URL
containing such characters may be represented as a quoted string within the ‘url()’ notation.
In order to create modular style sheets that are not dependent on the
absolute location of a resource, authors should use relative URIs.
Relative URIs (as defined in [URI]) are resolved to full URIs using a
base URI. RFC 3986, section 3, defines the normative algorithm
for this process. For CSS style sheets, the base URI is that of the style
sheet, not that of the source document.
When a <url> appears in the
computed value of a property, it is resolved to an absolute URL, as
described in the preceding paragraph.
For example, suppose the following rule:
body { "tile.png") }
is located in a style sheet designated by the URL:
www.example.org/style/basic.css
The background of the source document's <body> will
be tiled with whatever image is described by the resource designated by
the URL:
www.example.org/style/tile.png
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. For
unrestricted values, UAs must support at least up to ±230;
unsupported values must be clamped to the closest supported value.
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. For
unrestricted values, UAs must support at least up to ±230;
unsupported values must be clamped to the closest supported value.
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. For
unrestricted values, UAs must support at least up to ±230%;
unsupported values must be clamped to the closest supported value.
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
|
‘em’
| font size of the element
|
‘ex’
| x-height of the element's font
|
‘ch’
| width of the "0" glyph in the element's font
|
‘rem’
| font size of the root element
|
‘vw’
| viewport's width
|
‘vh’
| viewport's height
|
‘vmin’
| minimum of the viewport's height and width
|
Child elements do not inherit the relative values as specified for their
parent; they inherit the computed values.
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-.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 { .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-percentage lengths: the ‘vw’, ‘vh’, ‘vmin’ units
The viewport-percentage lengths are relative to the size of the initial
containing block. When the height or width of the viewport is changed,
they are scaled accordingly.
- vw unit
- Equal to 1% 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 { vw }
- vh unit
- Equal to 1% of the height of the initial containing block.
- vmin unit
- Equal to the smaller of ‘
vw’ or ‘vh’.
Note that Paged Media defines how the initial containing
block transforms across varying page widths. This also affects these
units.
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
|
‘cm’
| centimeters
|
‘mm’
| millimeters
|
‘in’
| inches; 1in is equal to 2.54cm
|
‘px’
| pixels; 1px is equal to 1/96th of 1in
|
‘pt’
| points; 1pt is equal to 1/72nd of 1in
|
‘pc’
| picas; 1pc is equal to 12pt
|
h1 { margin: 0.5in } /* inches */
h2 { line-cm } /* centimeters */
h3 { word-spacing: 4mm } /* millimeters */
h4 { pt } /* points */
h4 { pc } /* picas */
p { px } /* 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 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).
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:
- deg
- Degrees. There are 360 degrees in a full circle.
- grad
- Gradians, also known as "gons" or "grades". There are 400 gradians in
a full circle.
- rad
- Radians. There are 2π radians in a full circle.
- turn
- 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:
- s
- Seconds.
