attr

A função attr() do CSS  é utilizada para se obter o valor de um determinado atributo do elemento selecionado e usá-lo na folha de estilo. Também pode ser utilizado em pseudo-elementos, nesse caso o valor do atributo no pseudo-elemento do elemento original é retornado.

/* Uso Simples */
attr(data-count);
attr(title);

/* Com o tipo */
attr(src url);
attr(data-count number);
attr(data-width px);

/* Com fallback */
attr(data-count number, 0);
attr(src url, '');
attr(data-width px, inherit);
attr(data-something, 'default');

Nota: A função attr() pode ser utilizada com qualquer propriedade CSS, mas o suporte para propriedades que não sejam content é experimental.

Sintaxe

Valores

nome-do-atributo
É o nome do atributo HTML do elemento referenciado no CSS.
<tipo-ou-unidade>
É uma palavra-chave representando tanto o tipo quanto o valor do atributo, ou sua unidade, assim como no HTML alguns atributos tem unidades implícitas. Se o uso de <tipo-ou-unidade> como um valor para determinado atributo for inválido, a expressão attr() será inválida também. Se omitida, o padrão será string. A lista de valores válidos é:
Palavra-chave Tipo Associado Comentário Valor Padrão
string <string> O valor do atributo é tratado como um CSS <string>. NÃO será reparado, e em particular os caracteres usados . An empty string.
cor <color> O valor do atributo é analisado como hash (3- ou 6-valores hash) ou a palavra-chave. Precisa ser um valor <string> CSS válido.
Os espaços do lead e trail são retirados.
corAtual
url <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.
The url about:invalid that points to a non-existent document with a generic error condition.
integer <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.
0, or, if 0 is not a valid value for the property, the property's minimum value.
number <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.
0, or, if 0 is not a valid value for the property, the property's minimum value.
length <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.
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 <length> 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.
0, or, if 0 is not a valid value for the property, the property's minimum value.
angle <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.
0deg, or, if 0deg is not a valid value for the property, the property's minimum value.
deg, grad, rad <angle> 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.
0deg, or, if 0deg is not a valid value for the property, the property's minimum value.
time <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.
0s, or, if 0s is not a valid value for the property, the property's minimum value.
s, ms <time> 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.
0s, or, if 0s is not a valid value for the property, the property's minimum value.
frequency <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. 0Hz, or, if 0Hz is not a valid value for the property, the property's minimum value.
Hz, kHz <frequency> 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.
0Hz, or, if 0Hz is not a valid value for the property, the property's minimum value.
% <percentage> 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.
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. The fallback value must be valid where attr() is used, even if it is not used, and must not contain another attr() expression. If attr() is not the sole component value of a property, its <fallback> value must be of the type defined by <type-or-unit>. If not set, CSS will use the default value defined for each <type-or-unit>.

Sintaxe formal

attr( <attr-name> <type-or-unit>? [, <attr-fallback> ]? )

where
<type-or-unit> = string | color | url | integer | number | length | angle | time | frequency | cap | ch | em | ex | ic | lh | rlh | rem | vb | vi | vw | vh | vmin | vmax | mm | Q | cm | in | pt | pc | px | deg | grad | rad | turn | ms | s | Hz | kHz | %

Exemplos

HTML

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

CSS

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

Resultado

Especificações

Especificação Status Comentário
CSS Values and Units Module Level 3
The definition of 'attr()' in that specification.
Candidata a Recomendação Added two optional parameters; can be used on all properties; may return other values than <string>. These changes are experimental and may be dropped during the CR phase if browser support is too small.
CSS Level 2 (Revision 1)
The definition of 'attr()' in that specification.
Recomendação Limited to the content property; always return a <string>.

Compatibilidade dos Navegadores

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
attr()Chrome Full support 2Edge Full support 12Firefox Full support 1IE Full support 8Opera Full support 9Safari Full support 3.1WebView Android Full support ≤37Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 10.1Safari iOS Full support 3.1Samsung Internet Android Full support Yes
<fallback>
Experimental
Chrome No support NoEdge No support NoFirefox No support NoIE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoFirefox Android No support NoOpera Android No support NoSafari iOS No support NoSamsung Internet Android No support No
<type-or-unit>
Experimental
Chrome No support NoEdge No support NoFirefox No support NoIE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoFirefox Android No support NoOpera Android No support NoSafari iOS No support NoSamsung Internet Android No support No

Legend

Full support  
Full support
No support  
No support
Experimental. Expect behavior to change in the future.
Experimental. Expect behavior to change in the future.