@counter-style - CSS: Cascading Style Sheets 编辑

The @counter-style CSS at-rule lets you define counter styles that are not part of the predefined set of styles. A @counter-style rule defines how to convert a counter value into a string representation.

@counter-style thumbs {
  system: cyclic;
  symbols: "\1F44D";
  suffix: " ";
}

ul {
  list-style: thumbs;
}

The initial version of CSS defined a set of useful counter styles. However, although more styles were added to this set of predefined styles over the years, this system proved too restrictive to fulfill the needs of worldwide typography. The @counter-style at-rule addresses this shortcoming in an open-ended manner, by allowing authors to define their own counter styles when the pre-defined styles aren't fitting their needs.

Syntax

Descriptors

Each @counter-style is identified by a name and has a set of descriptors.

system

Specifies the algorithm to be used for converting the integer value of a counter to a string representation.

negative

Lets the author specify symbols to be appended or prepended to the counter representation if the value is negative.

prefix

Specifies a symbol that should be prepended to the marker representation. Prefixes are added to the representation in the final stage, so in the final representation of the counter, it comes before the negative sign.

suffix

Specifies, similar to the prefix descriptor, a symbol that is appended to the marker representation. Suffixes come after the marker representation.

range

Defines the range of values over which the counter style is applicable. If a counter style is used to represent a counter value outside of its ranges, the counter style will drop back to its fallback style.

pad

Is used when you need the marker representations to be of a minimum length. For example if you want the counters to start at 01 and go through 02, 03, 04 etc, then the pad descriptor is to be used. For representations larger than the specified pad value, the marker is constructed as normal.

fallback

Specifies a system to fall back into if either the specified system is unable to construct the representation of a counter value or if the counter value outside the specified range. If the specified fallback also fails to represent the value, then the fallback style's fallback is used, if one is specified. If there are either no fallback systems described or if the chain of fallback systems are unable to represent a counter value, then it will ultimately fall back to the decimal style.

symbols

Specifies the symbols that are to be used for the marker representations. Symbols can contain string, images or custom identifiers. How the symbols are used to construct the marker representation is up to the algorithm specified in the system descriptor. For example, if the system specified is fixed, then each of the N symbols specified in the descriptor will be used to represent the first N counter symbols. Once the specified set of symbols have exhausted, the fallback style will be used for the rest of the list.

The below @counter-style rule uses images instead of character symbols. Image values for symbols is currently an 'at risk' feature, and is not implemented in any browser.

@counter-style winners-list {
  system: fixed;
  symbols: url(gold-medal.svg) url(silver-medal.svg) url(bronze-medal.svg);
  suffix: " ";
}

additive-symbols

While the symbols specified in the symbols descriptor is used for constructing marker representation by most algorithms, some systems such as 'additive' rely on additive tuples described in this descriptor. Each additive tuple consists of a counter symbol and a non negative integer weight. The additive tuples must be specified in the descending order of their weights.

speak-as

Describes how to read out the counter style in speech synthesizers, such as screen readers. For example, the value of the marker symbol can be read out as numbers or alphabets for ordered lists or as audio cues for unordered lists, based on the value of this descriptor.

Formal syntax

@counter-style <counter-style-name> {
  [ system: <counter-system>; ] ||
  [ symbols: <counter-symbols>; ] ||
  [ additive-symbols: <additive-symbols>; ] ||
  [ negative: <negative-symbol>; ] ||
  [ prefix: <prefix>; ] ||
  [ suffix: <suffix>; ] ||
  [ range: <range>; ] ||
  [ pad: <padding>; ] ||
  [ speak-as: <speak-as>; ] ||
  [ fallback: <counter-style-name>; ]
}
where 
<counter-style-name> = <custom-ident>

Examples

Specifying symbols with counter-style

@counter-style circled-alpha {
  system: fixed;
  symbols: Ⓐ Ⓑ Ⓒ Ⓓ Ⓔ Ⓕ Ⓖ Ⓗ Ⓘ Ⓙ Ⓚ Ⓛ Ⓜ Ⓝ Ⓞ Ⓟ Ⓠ Ⓡ Ⓢ Ⓣ Ⓤ Ⓥ Ⓦ Ⓧ Ⓨ Ⓩ;
  suffix: " ";
}

The above counter style rule can be applied to lists like this:

.items {
  list-style: circled-alpha;
}

Which will produce lists like this:

Ⓐ One
Ⓑ Two
Ⓒ Three
Ⓓ Four
Ⓔ FIve
...
...
Ⓨ Twenty Five
Ⓩ Twenty Six
27 Twenty Seven
28 Twenty Eight
29 Twenty Nine
30 Thirty

See more examples on the demo page.

Specifications

Browser compatibility

BCD tables only load in the browser

Quantum CSS notes

• Gecko allowed none as a value of the system and fallback descriptors of @counter-style, which as per spec should result in an invalid at-rule. Firefox's new parallel CSS engine (also known as Quantum CSS or Stylo, released in Firefox 57) fixed this (bug 1377414).

See also

• list-style, list-style-image, list-style-position, list-style-type
• symbols(), the functional notation creating anonymous counter styles.
• CSS counter() and counters() functions
• Counter style demo (code)