system - CSS: Cascading Style Sheets 编辑

The system descriptor specifies the algorithm to be used for converting the integer value of a counter to a string representation. It is used in a @counter-style to define the behavior of the defined style.

If the algorithm specified in the system descriptor is unable to construct the representation for a particular counter value, then that value's representation will be constructed using the fallback system provided.

Syntax

/* Keyword values */
system: cyclic;
system: numeric;
system: alphabetic;
system: symbolic;
system: additive;
system: fixed;

/* Combined values */
system: fixed 3;
system: extends decimal;

This may take one of three forms:

• One of the keyword values cyclic, numeric, alphabetic, symbolic, additive, or fixed.
• The keyword value fixed along with an integer.
• The keyword value or extends along with a @counter-style name.

cyclic

Cycles through the list of symbols provided. Once the end of the list of symbols is reached, it will loop back to the beginning and start over. This system is useful for simple bullet styles with just one symbol, or for styles having multiple symbols. At least one symbol must be specified in the symbols descriptor, or the counter style is not valid.

fixed

Defines a finite set of symbols are specified. Once the system has looped through all the specified symbols, it will fall back. This system is useful in cases where the counter values are finite. At least one symbol must be specified in the symbols descriptor or the counter style is not valid. Also an optional <integer> can be specified after the system, as the value of the first symbol. If this integer is omitted, value of the first integer is taken as 1.

symbolic

Cycles through the provided list of symbols. On each successive pass through the cycle, the symbols used for the counter representation are doubled, tripled, and so on. For example, if the original symbols provided were "◽" and "◾", on each successive pass, they will become "◽◽" and "◾◾", "◽◽◽" and "◾◾◾" and so on. At least one symbol must be specified in the symbols descriptor or the counter style is not valid. This counter system works for positive counter values only.

alphabetic

Interprets the specified symbols as digits, to an alphabetic numbering system. If the characters "a" to "z" are specified as symbols in a counter style, with the alphabetic system, then the first 26 counter representations will be "a", "b" upto "z". Until this point, the behavior is the same as that of the symbolic system, described above. However, after "z", it will continue as "aa", "ab", "ac"…

The symbols descriptor must contain at least two symbols or the counter style is not valid. The first counter symbol provided in the symbols descriptor is interpreted as 1, the next as 2, and so on. This system is also defined strictly over positive counter values.

numeric

Interprets the counter symbols as digits in a place-value numbering system. The numeric system is similar to the alphabetic system, described above. The main difference is that in the alphabetic system, the first counter symbol given in the symbols descriptor is interpreted as 1, the next as 2, and so on. However, in the numeric system, the first counter symbol is interpreted as 0, the next as 1, then 2, and so on.

At least two counter symbols must be specified in the symbols descriptor or the counter style is not valid.

additive

Used to represent "sign-value" numbering systems, such as Roman numerals, which rather than reuse digits in different positions to obtain different values, define additional digits for larger values. The value of a number in such a system can be found out by adding the digits in the number.

An additional descriptor called additive-symbols must be specified with at least one additive tuple, or else the counter style rule will not be valid. An additive tuple is similar to a composite counter symbol, which is made up of two parts: a normal counter symbol and a non-negative integer weight. The additive tuples must be specified in the descending order of their weights or the system is invalid.

extends

Allows authors to use the algorithm of another counter style, but alter its other aspects. If a counter style rule is using the extends system, any unspecified descriptors, and their values will be taken from the extended counter style specified. If the specified counter style name in extends, is not a currently defined counter style name, it will instead extend from the decimal counter style.

It must not contain a symbols or additive-symbols descriptor, or else the counter style rule is invalid. If one or more counter styles defenitions form a cycle with their extends values, the browser will treat all the participating counter styles as extending from the decimal style.

Formal definition

Formal syntax

cyclic | numeric | alphabetic | symbolic | additive | [ fixed <integer>? ] | [ extends <counter-style-name> ]
where 
<counter-style-name> = <custom-ident>

Examples

Cyclic counter

If your browser supports it, this example will render a list like this:
◉ One
◉ Two
◉ Three

CSS

<ul>
  <li>One</li>
  <li>Two</li>
  <li>Three</li>
</ul>

@counter-style fisheye {
 system: cyclic;
 symbols: ◉;
 suffix: " ";
}

ul {
 list-style: fisheye;
}

Result

Fixed counter

If your browser supports it, this example will render a list like this:
➀ One
➁ Two
➂ Three
4 Four
5 Five

CSS

<ul>
 <li>One</li>
 <li>Two</li>
 <li>Three</li>
 <li>Four</li>
 <li>Five</li>
</ul>

@counter-style circled-digits {
 system: fixed;
 symbols: ➀ ➁ ➂;
 suffix: ' ';
}

ul {
 list-style: circled-digits;
}

Result

Symbolic counter

If your browser supports it, this example will render a list like this:
   a. One
   b. Two
   c. Three
 aa. Four
 bb. Five
 cc. Six
aaa. Seven
bbb. Eight

CSS

<ul>
 <li>One</li>
 <li>Two</li>
 <li>Three</li>
 <li>Four</li>
 <li>Five</li>
 <li>Six</li>
 <li>Seven</li>
 <li>Eight</li>
</ul>

@counter-style abc {
 system: symbolic;
 symbols: a b c;
 suffix: ". ";
}

ul {
 list-style: abc;
}

Result

Alphabetic counter

If your browser supports it, this example will render a list like this:
  a. One
  b. Two
  c. Three
aa. Four
ab. Five
ac. Six
ba. Seven
bb. Seven

CSS

<ul>
 <li>One</li>
 <li>Two</li>
 <li>Three</li>
 <li>Four</li>
 <li>Five</li>
 <li>Six</li>
 <li>Seven</li>
 <li>Eight</li>
</ul>

@counter-style abc {
 system: alphabetic;
 symbols: a b c;
 suffix: ". ";
}

ul {
 list-style: abc;
}

Result

Numeric counter

If your browser supports it, this example will render a list like this:

  b. One
  c. Two
ba. Three
bb. Four
bc. Five
ca. Six
cb. Seven
cc. Eight

The first symbol provided in the symbols descriptor is interpreted as 0 here.

CSS
 

<ul>
 <li>One</li>
 <li>Two</li>
 <li>Three</li>
 <li>Four</li>
 <li>Five</li>
 <li>Six</li>
 <li>Seven</li>
 <li>Eight</li>
</ul>

@counter-style abc {
 system: numeric;
 symbols: a b c;
 suffix: ". ";
}

ul {
 list-style: abc;
}

Result

Numeric counter with numeric symbols

As shown in the following example, if digits from 0 to 9 are specified as symbols, this counter style will render symbols same as the decimal counter style.

CSS

<ul class="list">
 <li>One</li>
 <li>Two</li>
 <li>Three</li>
 <li>Four</li>
 <li>Five</li>
 <li>Six</li>
 <li>Seven</li>
 <li>Eight</li>
 <li>Nine</li>
 <li>Ten</li>
</ul>

@counter-style numbers {
 system: numeric;
 symbols: "0" "1" "2" "3" "4" "5" "6" "7" "8" "9";
 suffix: ".";
}

ul {
 list-style: numbers;
}

Result

Additive counter

This example renders a list using Roman numerals. Notice that a range is specified. This is because the representation will produce correct Roman numerals only until the counter value of 3999. Once outside of the range, the rest of the counter representations will be based on the decimal style, which is the fall back. If you need to represent counter values as Roman numerals, you could use either one of  the predefined counter styles, upper-roman or lower-roman, rather than recreating the rule yourself.

HTML

<ul class="list">
 <li>One</li>
 <li>Two</li>
 <li>Three</li>
 <li>Four</li>
 <li>Five</li>
</ul>

CSS

@counter-style upper-roman {
 system: additive;
 range: 1 3999;
 additive-symbols: 1000 M, 900 CM, 500 D, 400 CD, 100 C, 90 XC, 50 L, 40 XL, 10 X, 9 IX, 5 V, 4 IV, 1 I;
}

ul {
 list-style: upper-roman;
}

Result

Extends example

This example will use the algorithm, symbols, and other properties of the lower-alpha counter style, but will remove the period ('.') after the counter representation, and enclose the characters in paranthesis; like (a), (b), etc.

HTML

<ul class="list">
 <li>One</li>
 <li>Two</li>
 <li>Three</li>
 <li>Four</li>
 <li>Five</li>
</ul>

CSS

@counter-style alpha-modified {
 system: extends lower-alpha;
 prefix: "(";
 suffix: ") ";
}

ul {
 list-style: alpha-modified;
}

Result

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• list-style, list-style-image, list-style-position
• symbols(), the functional notation creating anonymous counter styles.