4 CSS2Syntax and basic data types

4.1 Syntax

This section describes a grammar (and forward-compatible parsing rules) common to any versionlevel of CSS (including CSS2).CSS 2.1). Future versionsupdates of CSS will adhere to this core syntax, although they may add additional syntactic constraints.

These descriptions are normative. They are also complemented by the normative grammar rules presented in Appendix DG.

In this specification, the expressions "immediately before" or "immediately after" mean with no intervening white space or comments.

4.1.1 Tokenization

All levels of CSS --— level 1, level 2, and any future levels --— use the same core syntax. This allows UAs to parse (though not completely understand) style sheets written in levels of CSS that didn'tdid not exist at the time the UAs were created. Designers can use this feature to create style sheets that work with older user agents, while also exercising the possibilities of the latest levels of CSS.

At the lexical level, CSS style sheets consist of a sequence of tokens. The list of tokens for CSS2CSS is as follows. The definitions use Lex-style regular expressions. Octal codes refer to ISO 10646 ([ISO10646]). As in Lex, in case of multiple matches, the longest match determines the token.

The macros in curly braces ({}) above are defined as follows:

Below is the core syntax for CSS. The sections that follow describe how to use it. Appendix DG describes a more restrictive grammar that is closer to the CSS level 2 language. Parts of style sheets that can be parsed according to this grammar but not according to the grammar in Appendix G are among the parts that will be ignored according to the rules for handling parsing errors.

stylesheet  : [ CDO | CDC | S | statement ]*;
statement   : ruleset | at-rule;
at-rule     : ATKEYWORD S* any* [ block | ';' S* ];
block       : '{' S* [ any | block | ATKEYWORD S* | ';' S* ]* '}' S*;
ruleset     : selector? '{' S* declaration? [ ';' S* declaration? ]* '}' S*;
selector    : any+;
declaration : property S* ':' S* value;
property    :  IDENT S*;IDENT;
value       : [ any | block | ATKEYWORD S* ]+;
any         : [ IDENT | NUMBER | PERCENTAGE | DIMENSION | STRING
              | DELIM | URI | HASH | UNICODE-RANGE | INCLUDES
              |  FUNCTION |DASHMATCH | ':' | FUNCTION S* [any|unused]* ')'
              | '('  any*S* [any|unused]* ')' | '['  any*S* [any|unused]* ']'
              ] S*;
unused      : block | ATKEYWORD S* | ';' S* | CDO S* | CDC S*;

The "unused" production is not used in CSS and will not be used by any future extension. It is included here only to help with error handling. (See 4.2 "Rules for handling parsing errors.")

COMMENT tokens do not occur in the grammar (to keep it readable), but any number of these tokens may appear anywhere betweenoutside other tokens. (Note, however, that a comment before or within the @charset rule disables the @charset.)

The token S in the grammar above stands for whitespacewhite space. Only the characters "space" (Unicode code 32),(U+0020), "tab" (9),(U+0009), "line feed" (10),(U+000A), "carriage return" (13),(U+000D), and "form feed" (12)(U+000C) can occur in whitespace.white space. Other space-like characters, such as "em-space" (8195)(U+2003) and "ideographic space" (12288),(U+3000), are never part of whitespace.white space.

The meaning of input that cannot be tokenized or parsed is undefined in CSS 2.1.

4.1.2 Keywords

Keywords have the form of identifiers. Keywords must not be placed between quotes ("..." or '...'). Thus,

red

is a keyword, but

"red"

is not. (It is a string.) Other illegal examples:

width: "auto";
border: "none";
 font-family: "serif";background: "red";

4.1.3 Characters4.1.2.1 Vendor-specific extensions

In CSS, identifiers may begin with '-' (dash) or '_' (underscore). Keywords and case The following rules always hold: All CSS style sheets are case-insensitiveproperty names beginning with -' or '_' are reserved for vendor-specific extensions. Such vendor-specific extensions should have one of the following formats:

'-' + vendor identifier + '-' + meaningful name
'_' + vendor identifier + '-' + meaningful name

-moz-box-sizing
-moz-border-radius
-wap-accesskey

An initial dash or underscore is guaranteed never to be used in a property or keyword by any current or future level of CSS. Thus typical CSS implementations may not recognize such properties and may ignore them according to the rules for handling parsing errors. However, because the initial dash or underscore is part of the grammar, CSS 2.1 implementers should always be able to use a CSS-conforming parser, whether or not they support any vendor-specific extensions.

Authors should avoid vendor-specific extensions

4.1.2.2 Informative Historical Notes

This section is informative.

At the time of writing, the following prefixes are known to exist:

4.1.3 Characters and case

The following rules always hold:

• All CSS syntax is case-insensitive within the ASCII range (i.e., [a-z] and [A-Z] are equivalent), except for parts that are not under the control of CSS. For example, the case-sensitivity of values of the HTML attributes "id" and "class", of font names, and of URIs lies outside the scope of this specification. Note in particular that element names are case-insensitive in HTML, but case-sensitive in XML.
• In CSS2,CSS, identifiers (including element names, classes, and IDs in selectors) can contain only the characters [a-zA-Z0-9] and ISO 10646 characters 161U+0080 and higher, plus the hyphen (-);(-) and the underscore (_); they cannot start with a hyphendigit, two hyphens, or a hyphen followed by a digit. TheyIdentifiers can also contain escaped characters and any ISO 10646 character as a numeric code (see next item). For instance, the identifier "B&W?" may be written as "B\&W\?" or "B\26 W\3F".

  Note that Unicode is code-by-code equivalent to ISO 10646 (see [UNICODE] and [ISO10646]).

• In CSS2,CSS 2.1, a backslash (\) character indicatescan indicate one of three types of character escapes .escape. Inside a CSS comment, a backslash stands for itself, and if a backslash is immediately followed by the end of the style sheet, it also stands for itself (i.e., a DELIM token).

  First, inside a string, a backslash followed by a newline is ignored (i.e., the string is deemed not to contain either the backslash or the newline). Outside a string, a backslash followed by a newline stands for itself (i.e., a DELIM followed by a newline).

  Second, it cancels the meaning of special CSS characters. Any character (except a hexadecimal digit)digit, linefeed, carriage return, or form feed) can be escaped with a backslash to remove its special meaning. For example, "\"" is a string consisting of one double quote. Style sheet preprocessors must not remove these backslashes from a style sheet since that would change the style sheet's meaning.

  Third, backslash escapes allow authors to refer to characters they can'tcannot easily put in a document. In this case, the backslash is followed by at most six hexadecimal digits (0..9A..F), which stand for the ISO 10646 ([ISO10646]) character with that number.number, which must not be zero. (It is undefined in CSS 2.1 what happens if a digit or letterstyle sheet does contain a character with Unicode codepoint zero.) If a character in the range [0-9a-fA-F] follows the hexadecimal number, the end of the number needs to be made clear. There are two ways to do that:

  1. with a space (or other whitespacewhite space character): "\26 B" ("&B") by providing exactly 6("&B"). In this case, user agents should treat a "CR/LF" pair (U+000D/U+000A) as a single white space character.
  2. by providing exactly 6 hexadecimal digits: "\000026B" ("&B")

  In fact, these two methods may be combined. Only one whitespacewhite space character is ignored after a hexadecimal escape. Note that this means that a "real" space after the escape sequence must itself eitherbe escaped ordoubled.

  If the number is outside the range allowed by Unicode (e.g., "\110000" is above the maximum 10FFFF allowed in current Unicode), the UA may replace the escape with the "replacement character" (U+FFFD). If the character is to be displayed, the UA should show a visible symbol, such as a "missing character" glyph (cf. 15.2, point 5).

• Note: Backslash escapes are always considered to be part of an identifier or a string (i.e., "\7B" is not punctuation, even though "{" is, and "\32" is allowed at the start of a class name, even though "2" is not).

  The identifier "te\st" is exactly the same identifier as "test".

4.1.4 Statements

A CSS style sheet, for any versionlevel of CSS, consists of a list of statements (see the grammar above). There are two kinds of statements: at-rules and rule sets. There may be whitespacewhite space around the statements.

In this specification, the expressions "immediately before" or "immediately after" mean with no intervening whitespace or comments.4.1.5 At-rules

At-rules start with an at-keyword, an '@' character followed immediately by an identifier (for example, '@import', '@page').

An at-rule consists of everything up to and including the next semicolon (;) or the next block, whichever comes first.

A CSS user agent that encounters an unrecognized at-rule must ignore the whole of the at-rule and continue parsing after it. CSS2CSS 2.1 user agents must ignore any '@import' rule that occurs inside a block or that doesn't precede all rule sets.after any non-ignored statement other than an @charset or an @import rule.

@import "subs.css";
h1 { color: blue }
@import "list.css";

@import "subs.css";
h1 { color: blue }

@import "subs.css";
@media print {
  @import "print-main.css";
  body { font-size: 10pt }
}
h1 {color: blue }

@import "subs.css";
@import "print-main.css" print;
@media print {
  body { font-size: 10pt }
}
h1 {color: blue }

4.1.6 Blocks

A block starts with a left curly brace ({) and ends with the matching right curly brace (}). In between there may be any characters,tokens, except that parentheses (( )), brackets ([ ])([ ]), and braces ({ }) must always occur in matching pairs and may be nested. Single (') and double quotes (") must also occur in matching pairs, and characters between them are parsed as a string. See Tokenization above for the definition of a string.

{ causta: "}" + ({7} * '\'') }

4.1.7 Rule sets, declaration blocks, and selectors

A rule set (also called "rule") consists of a selector followed by a declaration block.

A declaration-block (also called a {}-block in the following text)declaration block starts with a left curly brace ({) and ends with the matching right curly brace (}). In between there must be a list of zero or more semicolon-separated (;) declarations.

The selector (see also the section on selectors) consists of everything up to (but not including) the first left curly brace ({). A selector always goes together with a {}-block.declaration block. When a user agent can'tcannot parse the selector (i.e., it is not valid CSS2),CSS 2.1), it must ignore the {}-blockselector and the following declaration block (if any) as well.

CSS2CSS 2.1 gives a special meaning to the comma (,) in selectors. However, since it is not known if the comma may acquire other meanings in future versionsupdates of CSS, the whole statement should be ignored if there is an error anywhere in the selector, even though the rest of the selector may look reasonable in CSS2.CSS 2.1.

h1, h2 {color: green }
h3, h4 & h5 {color: red }
h6 {color: black }

p[example="public class foo\
{\
    private int x;\
\
    foo(int x) {\
        this.x = x;\
    }\
\
}"] { color: red }

4.1.8 Declarations and properties

A declaration is either empty or consists of a property name, followed by a colon (:), followed by a property value. Around each of these there may be whitespacewhite space.

Because of the way selectors work, multiple declarations for the same selector may be organized into semicolon (;) separated groups.

h1 { font-weight: bold }
h1 { font-size:  12pt12px }
h1 { line-height:  14pt14px }
h1 { font-family: Helvetica }
h1 { font-variant: normal }
h1 { font-style: normal }

h1 {
  font-weight: bold;
  font-size:  12pt;12px;
  line-height:  14pt;14px;
  font-family: Helvetica;
  font-variant: normal;
  font-style: normal
}

A property name is an identifier. Any charactertoken may occur in the value, butproperty value. Parentheses ("( )"), brackets ("[ ]"), braces ("{ }"), single quotes (')('), and double quotes (") must come in matching pairs, and semicolons not in strings must be escaped. Parentheses, brackets, and braces may be nested. Inside the quotes, characters are parsed as a string.

The syntax of values is specified separately for each property, but in any case, values are built from identifiers, strings, numbers, lengths, percentages, URIs, colors, angles, times, and frequencies.etc.

A user agent must ignore a declaration with an invalid property name or an invalid value. Every CSS2CSS property has its own syntactic and semantic restrictions on the values it accepts.

h1 { color: red; font-style: 12pt }  /* Invalid value: 12pt */
p { color: blue;  font-vendor: any;  /* Invalid prop.: font-vendor */
    font-variant: small-caps }
em em { font-style: normal }

h1 { color: red; }
p { color: blue;  font-variant: small-caps }
em em { font-style: normal }

4.1.9 Comments

Comments begin with the characters "/*" and end with the characters "*/". They may occur anywhere betweenoutside other tokens, and their contents have no influence on the rendering. Comments may not be nested.

CSS also allows the SGML comment delimiters ("<!--" and "-->") in certain places,places defined by the grammar, but they do not delimit CSS comments. They are permitted so that style rules appearing in an HTML source document (in the STYLE element) may be hidden from pre-HTML 3.2 user agents. See the HTML 4.04 specification ( [HTML40][HTML4]) for more information.

4.2 Rules for handling parsing errors

In some cases, user agents must ignore part of an illegal style sheet. This specification defines ignore to mean that the user agent parses the illegal part (in order to find its beginning and end), but otherwise acts as if it had not been there. CSS 2.1 reserves for future updates of CSS all property:value combinations and @-keywords that do not contain an identifier beginning with dash or underscore. Implementations must ignore such combinations (other than those introduced by future updates of CSS).

To ensure that new properties and new values for existing properties can be added in the future, user agents are required to obey the following rules when they encounter the following scenarios:

• Unknown properties. User agents must ignore a declaration with an unknown property. For example, if the style sheet is:

  h1 { color: red; rotation: 70minutes }
  

  the user agent will treat this as if the style sheet had been

  
  h1 { color: red }
  

• Illegal values. User agents must ignore a declaration with an illegal value. For example:

  
  img { float: left }       /* correct  CSS2CSS 2.1 */
  img { float: left here }  /* "here" is not a value of 'float' */
  img { background: "red" } /* keywords cannot be quoted  in CSS2*/
  img { border-width: 3 }   /* a unit must be specified for length values */
  

  A CSS2CSS 2.1 parser would honor the first rule and ignore the rest, as if the style sheet had been:

  
  img { float: left }
  img { }
  img { }
  img { }
  

  A user agent conforming to a future CSS specification may accept one or more of the other rules as well.

• Invalid at-keywords.Malformed declarations. User agents must ignore an invalid at-keyword together with everything following it, up to and includinghandle unexpected tokens encountered while parsing a declaration by reading until the next semicolon (;) or block ({...}), whichever comes first. Forend of the declaration, while observing the rules for matching pairs of (), [], {}, "", and '', and correctly handling escapes. For example, a malformed declaration may be missing a property name, colon (:), or property value. The following are all equivalent:

  
  p { color:green }
  p { color:green; color }  /* malformed declaration missing ':', value */
  p { color:red;   color; color:green }  /* same with expected recovery */
  p { color:green; color: } /* malformed declaration missing value */
  p { color:red;   color:; color:green } /* same with expected recovery */
  p { color:green; color{;color:maroon} } /* unexpected tokens { } */
  p { color:red;   color{;color:maroon}; color:green } /* same with recovery */
  

• Malformed statements. User agents must handle unexpected tokens encountered while parsing a statement by reading until the end of the statement, while observing the rules for matching pairs of (), [], {}, "", and '', and correctly handling escapes. For example, a malformed statement may contain an unexpected closing brace or at-keyword. E.g., the following lines are all ignored:

  p @here {color: red}     /* ruleset with unexpected at-keyword "@here" */
  @foo @bar;               /* at-rule with unexpected at-keyword "@bar" */
  }} {{ - }}               /* ruleset with unexpected right brace */
  ) ( {} ) p {color: red } /* ruleset with unexpected right parenthesis */
  

• At-rules with unknown at-keywords. User agents must ignore an invalid at-keyword together with everything following it, up to the end of the block that contains the invalid at-keyword, or up to and including the next semicolon (;), or up to and including the next block ({...}), whichever comes first. For example, consider the following:

  
  @three-dee {
    @background-lighting {
      azimuth: 30deg;
      elevation: 190deg;
    }
    h1 { color: red }
  }
  h1 { color: blue }
  

  The '@three-dee' at-rule is not part of CSS2.CSS 2.1. Therefore, the whole at-rule (up to, and including, the third right curly brace) is ignored. A CSS2CSS 2.1 user agent ignores it, effectively reducing the style sheet to:

  
  h1 { color: blue }
  

  4.3 Values 4.3.1 Integers and real numbers Some value types may have integer values (denoted by <integer> ) or real number values (denoted by <number> ). Real numbers and integers are specified in decimal notation only. An <integer> consists of one or more digits "0" to "9". A <number> can either beSomething inside an at-rule that is ignored because it is invalid, such as an invalid declaration within an @media-rule, does not make the entire at-rule invalid.

• Unexpected end of style sheet.

  User agents must close all open constructs (for example: blocks, parentheses, brackets, rules, strings, and comments) at the end of the style sheet. For example:

  
    @media screen {
      p:before { content: 'Hello
  

  would be treated the same as:

  
    @media screen {
      p:before { content: 'Hello'; }
    }
  

  in a conformant UA.

• Unexpected end of string.

  User agents must close strings upon reaching the end of a line (i.e., before an unescaped line feed, carriage return or form feed character), but then drop the construct (declaration or rule) in which the string was found. For example:

  
        p {
          color: green;
          font-family: 'Courier New Times
          color: red;
          color: green;
        }
  

  ...would be treated the same as:

  
        p { color: green; color: green; }
  

  ...because the second declaration (from 'font-family' to the semicolon after 'color: red') is invalid and is dropped.

• See also Rule sets, declaration blocks, and selectors for parsing rules for declaration blocks.

4.3 Values

4.3.1 Integers and real numbers

Some value types may have integer values (denoted by <integer>) or real number values (denoted by <number>). Real numbers and integers are specified in decimal notation only. An <integer> consists of one or more digits "0" to "9". A <number> can either be an <integer>, or it can be zero or more digits followed by a dot (.) followed by one or more digits. Both integers and real numbers may immediately be preceded by a "-" or "+" to indicate the sign. -0 is equivalent to 0 and is not a negative number.

Note that many properties that allow an integer or real number as a value actually restrict the value to some range, often to a non-negative value.

4.3.2 Lengths

Lengths refer to horizontal or verticaldistance measurements.

The format of a length value (denoted by <length> in this specification) is an optional sign character ('+' or '-', with '+' being the default) immediately followed bya <number> (with or without a decimal point) immediately followed by a unit identifier (e.g., px, deg,em, etc.). After the '0'a zero length, the unit identifier is optional.

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

If a negative length value is set on a property that does not allow negative length values, the declaration is ignored.

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. Relative length units specify a length relative to another length property. Style sheets that use relative units willcan more easily scale from one mediumoutput environment to another (e.g., from a computer display to a laser printer).another.

Relative units are:

• em: the 'font-size' of the relevant font
• ex: the 'x-height' of the relevant font

h1 { margin: 0.5em }      /* em */
h1 { margin: 1ex }        /* ex */

P { font-size: 12px } /* px */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. It may be used for vertical or horizontal measurement. (This unit is also sometimes called the quad-width in typographic texts.)

The 'ex' unit is defined by the font's 'x-height' .element's first available font. The x-heightexception is when 'ex' occurs in the value of the 'font-size' property, in which case it refers to the 'ex' of the parent element.

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 don'tdo 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 should be used.

h1 { line-height: 1.2em }

h1 { font-size: 1.2em }

When specified for the root of the document tree (e.g., "HTML" in HTML), 'em' and 'ex' refer to the property's initial value.

Pixel units areChild elements do not inherit the relative tovalues specified for their parent; they inherit the resolution ofcomputed values.

body {
  font-size: 12px;
  text-indent: 3em;  /* i.e., 36px */
}
h1 { font-size: 15px }

Absolute length units are fixed in relation to each other. They are mainly useful when the reference pixel beoutput environment is known. The visual angle of one pixel on a device with a pixel densityabsolute units consist of 90dpi and a distance fromthe reader of an arm's length. For a nominal arm's length of 28 inches,physical units (in, cm, mm, pt, pc) and the visual angle is therefore about 0.0227 degrees. For reading at arm's length, 1px thus corresponds to about 0.28 mm (1/90 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 0.21 mm. 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 71 cm (28 inch) results in a px of 0.28 mm, while a reading distance of 3.5 m (12 feet) requires apx of 1.4 mm.     [D] 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).     [D] Child elements do not inherit the relative values specified for their parent; they (generally) inherit the computed values . Example(s): In the following rules, the computed 'text-indent' value of H1 elements will be 36pt, not 45pt, if H1 is a child of the BODY element. BODY { font-size: 12pt; text-indent: 3em; /* i.e., 36pt */ } H1 { font-size: 15pt } Absolute length units are only useful when the physical properties of the output medium are known. The absolute units are:unit:

• in: inches -- 1 inch— 1in is equal to 2.54 centimeters.2.54cm.
• cm: centimeters
• mm: millimeters
• pt: points --— the points used by CSS2CSS are equal to 1/72th1/72nd of an inch.1in.
• pc: picas -- 1 pica— 1pc is equal to 12 points. Example(s): 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. 4.3.3 Percentages The format of a percentage value (denoted by <percentage> in this specification)12pt.
• px: pixel units — 1px is an optional sign character ('+' or '-', with '+' being the default) immediately followed by a <number> immediately followed by '%'. Percentage values are always relativeequal to another value,0.75pt.

For examplea length. Each property that allows percentages also definesCSS device, these dimensions are either anchored (i) by relating the valuephysical units to whichtheir physical measurements, or (ii) by relating the percentage refers.pixel unit to the value may be that of another propertyreference pixel. For print media and similar high-resolution devices, the same element, a property for an ancestor element, or a valueanchor unit should be one of the formatting context (e.g., the width of a containing block ). When a percentage value is setstandard physical units (inches, centimeters, etc). For a property of the root elementlower-resolution devices, and devices with unusual viewing distances, it is recommended instead that the percentageanchor unit be the pixel unit. For such devices it is defined as referringrecommended that the pixel unit refer to the inherited valuewhole number of some property,device pixels that best approximates the resultant valuereference pixel.

Note that if the anchor unit is the percentage timespixel unit, the initial valuephysical 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 property. Example(s): Since child elements (generally) inherit the computed valuesthis definition of their parent, inthe following example,pixel unit and the childrenphysical units differs from previous versions of the P element will inherit a valueCSS. In particular, in previous versions of 12pt for 'line-height' , notCSS the percentage value (120%): P { font-size: 10pt } P { line-height: 120% } /* 120% of 'font-size' */ 4.3.4 URL + URN = URI URLs (Uniform Resource Locators, see [RFC1738]pixel unit and [RFC1808] ) providethe address ofphysical units were not related by a resourcefixed 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 Web. An expected new wayassumption of identifying resources is called URN (Uniform Resource Name). Together they are called URIs (Uniform Resource Identifiers, see [URI] ). This specification uses96dpi, and breaking that assumption breaks the term URI. URI values in this specification are denoted by <uri> .content.)

The functional notation used to designate URIs in property valuesreference pixel is "url()", as in: Example(s): BODY { background: url("http://www.bg.com/pinkish.gif") }the formatvisual angle of one pixel on a URI value is 'url(' followed by optional whitespace followed by an optional single quote (') or double quote (") character followed bydevice with a pixel density of 96dpi and a distance from the URI itself, followed byreader of an optional single quote (') or double quote (") character followed by optional whitespace followed by ')'.arm's length. For a nominal arm's length of 28 inches, the two quote characters must bevisual 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 same. Example(s): An example without quotes: LI { list-style: url(http://www.redballs.com/redball.png) disc } Parentheses, commas, whitespace characters, single quotes (') and double quotes (") appearingimage 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 URI must be escaped withreference pixel of 0.26 mm, while a backslash: '\(', '\)', '\,'. Depending on the typereading distance of URI, it might also be possible to write3.5 m (12 feet) results in a reference pixel of 1.3 mm.

This second image illustrates the above characters as URI-escapes (where "(" = %28, ")" = %29, etc.) as describedeffect of a device's resolution on the pixel unit: an area of 1px by 1px is covered by a single dot in [URI] .a low-resolution device (e.g. a typical computer display), while the same area is covered by 16 dots in order to create modular style sheets that are not dependent ona higher resolution device (such as a printer).

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 */

4.3.3 Percentages

The absolute locationformat of a resource, authors may use relative URIs. Relative URIs (as definedpercentage value (denoted by <percentage> in [RFC1808] )this specification) is a <number> immediately followed by '%'.

Percentage values are resolvedalways relative to full URIs usinganother value, for example a base URI. RFC 1808, section 3,length. Each property that allows percentages also defines the normative algorithm for this process. For CSS style sheets,value to which the base URI is that ofpercentage refers. The style sheet, notvalue may be that of the source document. Example(s):another property for example, supposethe following rule: BODY { background: url("yellow") } is located insame element, a style sheet designated byproperty for an ancestor element, or a value of the URI: http://www.myorg.org/style/basic.cssformatting context (e.g., the backgroundwidth of the source document's BODY will be tiled with whatever imagea containing block). When a percentage value is described by the resource designated by the URI http://www.myorg.org/style/yellow User agents may vary in how they handle URIs that designate unavailable or inapplicable resources. 4.3.5 Counters Counters are denoted by identifiers (seeset for a property of the 'counter-increment'root element and 'counter-reset' properties). To referthe percentage is defined as referring to the inherited value of a counter,some property, the notation 'counter(<identifier>)' or 'counter(<identifier>, <list-style-type>)'resultant value is used.the default style is 'decimal'. To refer to a sequence of nested counters ofpercentage times the same name,initial value of that property.

p { font-size: 10px }
p { line-height: 120% }  /* 120% of  any 'counter-reset','font-size' */

4.3.4 URLs and URIs

URI values (Uniform Resource Identifiers, see [RFC3986], which includes URLs, URNs, etc) in this specification are assumed to have been reset to 0denoted by a 'counter-reset' on<uri>. The root element. 4.3.6 Colors A <color> is either a keyword or a numerical RGB specification. The list of keyword color names is: aqua, black, blue, fuchsia, gray, green, lime, maroon, navy, olive, purple, red, silver, teal, white, and yellow. These 16 colors are defined in HTML 4.0 ( [HTML40] ). In addition to these color keywords, users may specify keywords that correspond to the colorsfunctional notation used by certain objectsto designate URIs in the user's environment. Please consult the section on system colors for more information. Example(s): BODY {color: black; background: white } H1 { color: maroon } H2 { color: olive } The RGB color modelproperty values is used in numerical color specifications. These examples all specify the same color:"url()", as in:

 EM { color: #f00 } /* #rgb */ EM { color: #ff0000 } /* #rrggbb */ EM { color: rgb(255,0,0) } /* integer range 0 - 255 */ EMbody {  color: rgb(100%, 0%, 0%)background: url("http://www.example.com/pinkish.png") }

/* float range 0.0% - 100.0% */The format of an RGBa URI value in hexadecimal notationis a '#' immediately'url(' followed by either three or six hexadecimal characters. The three-digit RGB notation (#rgb) is converted into six-digit form (#rrggbb) by replicating digits, not by adding zeros. For example, #fb0 expands to #ffbb00. This ensures thatoptional white (#ffffff) can be specified with the short notation (#fff) and removes any dependencies on the color depth of the display. The format of an RGB value in the functional notation is 'rgb('space followed by a comma-separated list of three numerical values (either three integer valuesan optional single quote (') or three percentage values)double quote (") character followed by ')'.the integer value 255 corresponds to 100%, and to FURI itself, followed by an optional single quote (') or FF indouble quote (") character followed by optional white space followed by ')'. The hexadecimal notation: rgb(255,255,255) = rgb(100%,100%,100%) = #FFF. Whitespacetwo quote characters are allowed around the numerical values. All RGB colors are specified inmust be the sRGB color space (see [SRGB] ). User agents may varysame.

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

Some characters appearing in the fidelity with which they represent these colors, but using sRGB providesan unambiguousunquoted URI, such as parentheses, white space characters, single quotes (') and objectively measurable definition of what the color should be, which candouble quotes ("), must be related to international standards (see [COLORIMETRY] ). Conforming user agents may limit their color-displaying efforts to performingescaped with a gamma-correction on them. sRGB specifiesbackslash so that the resulting URI value is a display gamma of 2.2 under specified viewing conditions. User agents should adjustURI token: '\(', '\)'.

Depending on the colors given in CSS such that, in combination with an output device's "natural" display gamma, an effective display gammatype of 2.2 is produced. SeeURI, it might also be possible to write the section on gamma correction for further details.above characters as URI-escapes (where "(" = %28, ")" = %29, etc.) as described in [RFC3986].

In CSS are affected; e.g., images are expectedorder to carry their own color information. Values outsidecreate modular style sheets that are not dependent on the device gamut should be clipped: the red, green, and blue values must be changedabsolute location of a resource, authors may use relative URIs. Relative URIs (as defined in [RFC3986]) are resolved to fall within the range supported byfull URIs using a base URI. RFC 3986, section 5, defines the device.normative algorithm for a typical CRT monitor, whose device gamutthis process. For CSS style sheets, the base URI is that of the same as sRGB,style sheet, not that of the three rules below are equivalent:source document.

body {  color: rgb(110%, 0%, 0%)background: url("yellow") }

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

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

User agents may vary in how they handle invalid URIs or '-', with '+' being the default) immediately followed by a <number> immediately followed by an angle unit identifier. Angle unitURIs that designate unavailable or inapplicable resources.

4.3.5 Counters

Counters are denoted by case-sensitive identifiers are: deg : degrees grad : grads rad : radians Angle values may be negative. They should be normalized(see the 'counter-increment' and 'counter-reset' properties). To refer to the range 0-360deg byvalue of a counter, the user agent. For example, -10deg and 350deg are equivalent. For example,notation 'counter(<identifier>)' or 'counter(<identifier>, <'list-style-type'>)', with optional white space separating the tokens, is used. The default style is 'decimal'.

To refer to a right anglesequence of nested counters of the same name, the notation is '90deg''counters(<identifier>, <string>)' or '100grad''counters(<identifier>, <string>, <'list-style-type'>)' with optional white space separating the tokens.

See "Nested counters and scope" in the chapter on generated content for how user agents must determine the value or '1.570796326794897rad'. 4.3.8 Times Timevalues (denoted by <time>of the counter. See the definition of counter values of the 'content' property for how it must convert these values to a string.

In CSS 2.1, the text) are used with aural style sheets . Their formatvalues of counters can only be referred to from the 'content' property. Note that 'none' is a <number> immediately followed bypossible <'list-style-type'>: 'counter(x, none)' yields an empty string.

p {counter-increment: par-num}
h1 {counter-reset: par-num}
p:before {content: counter(par-num, upper-roman) ". "}

4.3.6 Colors

A <color> is either be written with double quotesa keyword or a numerical RGB specification.

The list of color keywords is: aqua, black, blue, fuchsia, gray, green, lime, maroon, navy, olive, orange, purple, red, silver, teal, white, and yellow. These 17 colors have the following values:

In addition to these color keywords, users may specify keywords that correspond to the colors used by certain objects in the user's environment. Please consult the section on system colors for more information.

body {color: black; background: white }
h1 { color: maroon }
h2 { color: olive }

The RGB color model is used in numerical color specifications. These examples all specify the same color:

em { color: #f00 }              /* #rgb */
em { color: #ff0000 }           /* #rrggbb */
em { color: rgb(255,0,0) }      
em { color: rgb(100%, 0%, 0%) } 

The format of an RGB value in hexadecimal notation is a '#' immediately followed by either three or six hexadecimal characters. The three-digit RGB notation (#rgb) is converted into six-digit form (#rrggbb) by replicating digits, not by adding zeros. For example, #fb0 expands to #ffbb00. This ensures that white (#ffffff) can be specified with single quotes. Double quotes cannot occur inside double quotes, unless escaped (as '\"'the short notation (#fff) and removes any dependencies on the color depth of the display.

The format of an RGB value in the functional notation is 'rgb(' followed by a comma-separated list of three numerical values (either three integer values or three percentage values) followed by ')'. The integer value 255 corresponds to 100%, and to F or FF in the hexadecimal notation: rgb(255,255,255) = rgb(100%,100%,100%) = #FFF. White space characters are allowed around the numerical values.

All RGB colors are specified in the sRGB color space (see [SRGB]). User agents may vary in the fidelity with which they represent these colors, but using sRGB provides an unambiguous and objectively measurable definition of what the color should be, which can be related to international standards (see [COLORIMETRY]).

Conforming user agents may limit their color-displaying efforts to performing a gamma-correction on them. sRGB specifies a display gamma of 2.2 under specified viewing conditions. User agents should adjust the colors given in CSS such that, in combination with an output device's "natural" display gamma, an effective display gamma of 2.2 is produced. Note that only colors specified in CSS are affected; e.g., images are expected to carry their own color information.

Values outside the device gamut should be clipped or mapped into the gamut when the gamut is known: the red, green, and blue values must be changed to fall within the range supported by the device. Users agents may perform higher quality mapping of colors from one gamut to another. For a typical CRT monitor, whose device gamut is the same as sRGB, the four rules below are equivalent:

em { color: rgb(255,0,0) }       /* integer range 0 - 255 */
em { color: rgb(300,0,0) }       /* clipped to rgb(255,0,0) */
em { color: rgb(255,-10,0) }     /* clipped to rgb(255,0,0) */
em { color: rgb(110%, 0%, 0%) }  /* clipped to rgb(100%,0%,0%) */

Other devices, such as printers, have different gamuts than sRGB; some colors outside the 0..255 sRGB range will be representable (inside the device gamut), while other colors inside the 0..255 sRGB range will be outside the device gamut and will thus be mapped.

4.3.7 Strings

Strings can either be written with double quotes or with single quotes. Double quotes cannot occur inside double quotes, unless escaped (e.g., as '\"' or as '\22'). Analogously for single quotes (e.g., "\'" or "\27").

"this is a 'string'"
"this is a \"string\""
'this is a "string"'
'this is a \'string\''

A string cannot directly contain a newline. To include a newline in a string, use an escape representing the line feed character in ISO-10646 (U+000A), such as "\A" or "\00000a". This character 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 (\). 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"] {/*...*/}

4.3.8 Unsupported Values

If a UA does not support a particular value, it should ignore that value when parsing style sheets, as if that value was an illegal value. For example:

  h3 {
    display: inline;
    display: run-in;
  }

A UA that supports the 'run-in' value for the 'display' property will accept the first display declaration and then "write over" that value with the second display declaration. A UA that does not support the 'run-in' value will process the first display declaration and ignore the second display declaration.

4.4 CSS style sheet representation

A CSS style sheet is a sequence of characters from the Universal Character Set (see [ISO10646]). For transmission and storage, these characters must be encoded by a character encoding that supports the set of characters available in US-ASCII (e.g., UTF-8, ISO 8859-x, SHIFT JIS, etc.). For a good introduction to character sets and character encodings, please consult the HTML 4 specification ([HTML4], chapter 5). See also the XML 1.0 specification ([XML10], sections 2.2 and 4.3.3, and Appendix F).

When a style sheet is embedded in another document, such as in the STYLE element or "style" attribute of HTML, the style sheet shares the character encoding of the whole document.

When a style sheet resides in a separate file, user agents must observe the following priorities when determining a style sheet's character encoding (from highest priority to lowest):

1. An HTTP "charset" parameter in a "Content-Type" field (or similar parameters in other protocols)
2. BOM and/or @charset (see below)
3. <link charset=""> or other metadata from the linking mechanism (if any)
4. charset of referring style sheet or document (if any)
5. Assume UTF-8

Authors using an @charset rule must place the rule at the very beginning of the style sheet, preceded by no characters. (If a byte order mark is appropriate for the encoding used, it may precede the @charset rule.)

After "@charset", authors specify the name of a character encoding (in quotes). For example:

@charset "ISO-8859-1";

@charset must be written literally, i.e., the 10 characters '@charset "' (lowercase, no backslash escapes), followed by the encoding name, followed by '";'.

The name must be a charset name as '\22'). Analogouslydescribed in the IANA registry. See [CHARSETS] for single quotes ("\'"a complete list of charsets. Authors should use the charset names marked as "preferred MIME name" in the IANA registry.

User agents must support at least the UTF-8 encoding.

If rule 1 above (an HTTP "charset" parameter or "\27"). Example(s): "this issimilar) yields a 'string'" "thischaracter encoding and it is one of UTF-8, UTF-16 or UTF-32, then a \"string\"" 'this isBOM, if any, at the start of the file overrides that character encoding, as follows:

If rule 1 yields a "string"' 'thischaracter encoding of UTF-16BE, UTF-16LE, UTF-32BE or UTF-32LE, then it is an error if the file starts with a \'string\'' A string cannot directly contain a newline . To includeBOM. A newline inCSS UA must recover by ignoring the specified encoding and using the table above.

Note that the fact that a string, useBOM at the escape "\A" (hexadecimalstart of a file is the line feed characteren error in Unicode, but representsUTF-16BE, UTF-16LE, UTF-32BE or UTF-32LE is specified by [UNICODE].

User agents must ignore any @charset rule not at the generic notionbeginning of "newline"the style sheet. When user agents detect the character encoding using the BOM and/or the @charset rule, they should follow the following rules:

• Except as specified in CSS). Seethese rules, all @charset rules are ignored.
• The 'content' property for an example. Itencoding is possible to break strings over several lines, for aesthetic or other reasons, but in such a casedetected based on the newline itself has to be escaped withstream of bytes that begins the style sheet. The following table gives a backslash (\).set of possibilities for instance,initial byte sequences (written in hexadecimal). The first row that matches the following two selectors are exactlybeginning of the same: Example(s): A[TITLE="a not s\ o very long title"] {/*...*/} A[TITLE="a not so very long title"] {/*...*/} 4.4 CSS document representation A CSSstyle sheet is a sequencegives the result of characters fromencoding detection based on the BOM and/or @charset rule. If no rows match, the Universal Character Set (see [ISO10646] ). For transmission and storage, these characters must be encoded by a characterencoding cannot be detected based on the BOM and/or @charset rule. The notation (...)* refers to repetition for which the best match is the one that supportsrepeats as few times as possible. The setbytes marked "XX" are those used to determine the name of characters availablethe encoding, by treating them, in US-ASCII (e.g., ISO 8859-x, SHIFT JIS, etc.). Forthe order given, as a good introductionsequence of ASCII characters. Bytes marked "YY" are similar, but need to character sets and character encodings, please consultbe transcoded into ASCII as noted. User agents may ignore entries in the HTML 4.0 specification ( [HTML40] , chapter 5), See alsotable if they do not support any encodings relevant to the XML 1.0 specification ( [XML10] , sections 2.2 and 4.3.3, and Appendix F. When a style sheet is embedded in another document, suchentry.

  Initial Bytes	Result
  EF BB BF 40 63 68 61 72 73 65 74 20 22 (XX)* 22 3B	as specified
  EF BB BF	UTF-8
  40 63 68 61 72 73 65 74 20 22 (XX)* 22 3B	as specified
  FE FF 00 40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 (00 XX)* 00 22 00 3B	as specified (with BE endianness if not specified)
  00 40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 (00 XX)* 00 22 00 3B	as specified (with BE endianness if not specified)
  FF FE 40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 00 (XX 00)* 22 00 3B 00	as specified (with LE endianness if not specified)
  40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 00 (XX 00)* 22 00 3B 00	as specified (with LE endianness if not specified)
  00 00 FE FF 00 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 (00 00 00 XX)* 00 00 00 22 00 00 00 3B	as specified (with BE endianness if not specified)
  00 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 (00 00 00 XX)* 00 00 00 22 00 00 00 3B	as specified (with BE endianness if not specified)
  00 00 FF FE 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 (00 00 XX 00)* 00 00 22 00 00 00 3B 00	as specified (with 2143 endianness if not specified)
  00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 (00 00 XX 00)* 00 00 22 00 00 00 3B 00	as specified (with 2143 endianness if not specified)
  FE FF 00 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 00 (00 XX 00 00)* 00 22 00 00 00 3B 00 00	as specified (with 3412 endianness if not specified)
  00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 00 (00 XX 00 00)* 00 22 00 00 00 3B 00 00	as specified (with 3412 endianness if not specified)
  FF FE 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 00 00 (XX 00 00 00)* 22 00 00 00 3B 00 00 00	as specified (with LE endianness if not specified)
  40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 00 00 (XX 00 00 00)* 22 00 00 00 3B 00 00 00	as specified (with LE endianness if not specified)
  00 00 FE FF	UTF-32-BE
  FF FE 00 00	UTF-32-LE
  00 00 FF FE	UTF-32-2143
  FE FF 00 00	UTF-32-3412
  FE FF	UTF-16-BE
  FF FE	UTF-16-LE
  7C 83 88 81 99 A2 85 A3 40 7F (YY)* 7F 5E	as specified, transcoded from EBCDIC to ASCII
  AE 83 88 81 99 A2 85 A3 40 FC (YY)* FC 5E	as specified, transcoded from IBM1026 to ASCII
  00 63 68 61 72 73 65 74 20 22 (YY)* 22 3B	as in the STYLE element or "style" attribute of HTML, the style sheet shares the character encoding of the whole document. When a style sheet resides in a separate file,specified, transcoded from GSM 03.38 to ASCII
  analogous patterns	User agents must observe the following priorities when determining a document's character encoding (from highest priority to lowest): An HTTP "charset" parameter in a "Content-Type" field.may support additional, analogous, patterns if they support encodings that are not handled by the @charset at-rule. Mechanisms ofpatterns here

• If the languageencoding is detected based on one of the referencing document (e.g.,entries in HTML,the "charset" attribute oftable above marked "as specified", the user agent ignores the LINK element). At most one @charset rule may appear in an externalstyle sheet --if it mustdoes not appear inparse an embedded style sheet -- and it must appearappropriate @charset rule at the very startbeginning of the document, not preceded by any characters. After "@charset", authors specify the namestream of a character encoding. The name must be a charset name as describedcharacters resulting from decoding in the IANA registry (See [IANA] . Also, see [CHARSETS] for a complete list of charsets). For example: Example(s): @charset "ISO-8859-1";chosen @charset. This specification does not mandate which character encodings a user agent must support. Note that reliance on theensures that:
  • @charset construct theoretically poses a problem since there is no a priori information on how it is encoded.rules should only function if they are in practice, however,the encodings in wide use onencoding of the Internetstyle sheet,
  • byte order marks are either based on ASCII, UTF-16, UCS-4, or (rarely) on EBCDIC. This means thatignored only in general, the initial byte values of a document enableencodings that support a user agent to detect thebyte order mark, and
  • encoding family reliably, which provides enough information to decode the @charset rule, whichnames cannot contain newlines.

User agents must ignore style sheets in turn determines the exact character encoding.unknown encodings.

4.4.1 Referring to characters not represented in a character encoding

A style sheet may have to refer to characters that cannot be represented in the current character encoding. These characters must be written as escaped references to ISO 10646 characters. These escapes serve the same purpose as numeric character references in HTML or XML documents (see [HTML40][HTML4], chapters 5 and 25).

The character escape mechanism should be used when only a few characters must be represented this way. If most of a documentstyle sheet requires escaping, authors should encode it with a more appropriate encoding (e.g., if the documentstyle sheet contains a lot of Greek characters, authors might use "ISO-8859-7" or "UTF-8").

Intermediate processors using a different character encoding may translate these escaped sequences into byte sequences of that encoding. Intermediate processors must not, on the other hand, alter escape sequences that cancel the special meaning of an ASCII character.

Conforming user agents must correctly map to UnicodeISO-10646 all characters in any character encodings that they recognize (or they must behave as if they did).

For example, a documentstyle sheet transmitted as ISO-8859-1 (Latin-1) cannot contain Greek letters directly: "κουρος" (Greek: "kouros") has to be written as "\3BA\3BF\3C5\3C1\3BF\3C2".

<SPAN  style="voice-family: D\FC rst">...</SPAN>style="font-family: L\FC beck">...</SPAN>

<SPAN  style="voice-family: D&#252;rst">...</SPAN>style="font-family: L&#252;beck">...</SPAN>