attr()

attribute-name

The name of an attribute on the HTML element referenced in the CSS.

<type-or-unit>

A keyword representing either the type of the attribute's value, or its unit, as in HTML some attributes have implicit units. If the use of <type-or-unit> as a value for the given attribute is invalid, the attr() expression will be invalid too. If omitted, it defaults to string. The list of valid values are:

string

The attribute value is treated as a CSS <string>. It is NOT reparsed, and in particular the characters are used as-is instead of CSS escapes being turned into different characters.

Default value: an empty string.

color

The attribute value is parsed as a hash (3- or 6-value hash) or a keyword. It must be a valid CSS <string> value. Leading and trailing spaces are stripped.

Default value: currentcolor.

url

The attribute value is parsed as a string that is used inside a CSS url() function. Relative URL are resolved relatively to the original document, not relatively to the style sheet. Leading and trailing spaces are stripped.

Default value: the URL about:invalid that points to a non-existent document with a generic error condition.

integer

The attribute value is parsed as a CSS <integer>. If it is not valid, that is not an integer or out of the range accepted by the CSS property, the default value is used. Leading and trailing spaces are stripped.

Default value: 0, or, if 0 is not a valid value for the property, the property's minimum value.

number

The attribute value is parsed as a CSS <number>. If it is not valid, that is not a number or out of the range accepted by the CSS property, the default value is used. Leading and trailing spaces are stripped.

Default value: 0, or, if 0 is not a valid value for the property, the property's minimum value.

length

The attribute value is parsed as a CSS <length> dimension, that is including the unit (e.g. 12.5em). If it is not valid, that is not a length or out of the range accepted by the CSS property, the default value is used. If the given unit is a relative length, attr() computes it to an absolute length. Leading and trailing spaces are stripped.

Default value: 0, or, if 0 is not a valid value for the property, the property's minimum value.

em, ex, px, rem, vw, vh, vmin, vmax, mm, cm, in, pt, or pc

The attribute value is parsed as a CSS <number>, that is without the unit (e.g. 12.5), and interpreted as a <length> with the specified unit. If it is not valid, that is not a number or out of the range accepted by the CSS property, the default value is used. If the given unit is a relative length, attr() computes it to an absolute length. Leading and trailing spaces are stripped.

Default value: 0, or, if 0 is not a valid value for the property, the property's minimum value.

angle

The attribute value is parsed as a CSS <angle> dimension, that is including the unit (e.g. 30.5deg). If it is not valid, that is not an angle or out of the range accepted by the CSS property, the default value is used. Leading and trailing spaces are stripped.

Default value: 0deg, or, if 0deg is not a valid value for the property, the property's minimum value.

deg, grad, rad

The attribute value is parsed as a CSS <number>, that is without the unit (e.g. 12.5), and interpreted as an <angle> with the specified unit. If it is not valid, that is not a number or out of the range accepted by the CSS property, the default value is used. Leading and trailing spaces are stripped.

Default value: 0deg, or, if 0deg is not a valid value for the property, the property's minimum value.

time

The attribute value is parsed as a CSS <time> dimension, that is including the unit (e.g. 30.5ms). If it is not valid, that is not a time or out of the range accepted by the CSS property, the default value is used. Leading and trailing spaces are stripped.

Default value: 0s, or, if 0s is not a valid value for the property, the property's minimum value.

s, ms

The attribute value is parsed as a CSS <number>, that is without the unit (e.g. 12.5), and interpreted as an <time> with the specified unit. If it is not valid, that is not a number or out of the range accepted by the CSS property, the default value is used. Leading and trailing spaces are stripped.

Default value: 0s, or, if 0s is not a valid value for the property, the property's minimum value.

frequency

The attribute value is parsed as a CSS <frequency> dimension, that is including the unit (e.g. 30.5kHz). If it is not valid, that is not a frequency or out of the range accepted by the CSS property, the default value is used.

Default value: 0Hz, or, if 0Hz is not a valid value for the property, the property's minimum value.

Hz, kHz

The attribute value is parsed as a CSS <number>, that is without the unit (e.g. 12.5), and interpreted as a <frequency> with the specified unit. If it is not valid, that is not a number or out of the range accepted by the CSS property, the default value is used. Leading and trailing spaces are stripped.

Default value: 0Hz, or, if 0Hz is not a valid value for the property, the property's minimum value.

%

The attribute value is parsed as a CSS <number>, that is without the unit (e.g. 12.5), and interpreted as a <percentage>. If it is not valid, that is not a number or out of the range accepted by the CSS property, the default value is used. If the given value is used as a length, attr() computes it to an absolute length. Leading and trailing spaces are stripped.

Default value: 0%, or, if 0% is not a valid value for the property, the property's minimum value.

<fallback>

The value to be used if the associated attribute is missing or contains an invalid value. If not set, CSS will use the default value defined for each <type-or-unit>.

<attr()> = 
  attr( <attr-name> <attr-type>? , <declaration-value>? )  

<attr-name> = 
  [ <ident-token> '|' ]? <ident-token>  

<attr-type> = 
  type( <syntax> )  |
  string            |
  <attr-unit>       

<syntax> = 
  '*'                                                 |
  <syntax-component> [ <syntax-combinator> <syntax-component> ]*  |
  <syntax-string>                                     

<syntax-component> = 
  <syntax-single-component> <syntax-multiplier>?  |
  '<' transform-list '>'                          

<syntax-combinator> = 
  '|'  

<syntax-string> = 
  <string>  

<syntax-single-component> = 
  '<' <syntax-type-name> '>'  |
  <ident>                     

<syntax-multiplier> = 
  '#'  |
  '+'  

<syntax-type-name> = 
  angle               |
  color               |
  custom-ident        |
  image               |
  integer             |
  length              |
  length-percentage   |
  number              |
  percentage          |
  resolution          |
  string              |
  time                |
  url                 |
  transform-function  

In this example, we prepend the value of the data-foo data-* global attribute to the contents of the <p> element.

HTML

<p data-foo="hello">world</p>

CSS

[data-foo]::before {
  content: attr(data-foo) " ";
}

Result

In this example, we set the CSS value of background-color to the value of the data-background data-* global attribute assigned to the <div> element.

HTML

<div class="background" data-background="lime">
  background expected to be red if your browser does not support advanced usage
  of attr()
</div>

CSS

.background {
  height: 100vh;
}

.background {
  background-color: red;
}

.background[data-background] {
  background-color: attr(data-background color, red);
}

Result

In this example, we append the value of data-browser data-* global attribute to the <p> element. If the data-browser attribute is missing from the <p> element, we append the fallback value of "Unknown".

HTML

<p data-browser="Firefox">My favorite browser is:</p>
<p>Your favorite browser is:</p>

CSS

p::after {
  content: " " attr(data-browser, "Unknown");
  color: tomato;
}

Result

• Attribute selectors
• HTML data-* attributes
• SVG data-* attributes