Returns the value of an attribute of the selected element.
Below is a blockquote element which contains a character inside a [data-*
attribute](http://stackoverflow.com/documentation/html/1182/html5-data-attribute/6305/definition-and-usage#t=201607221052072794425) which CSS can use (e.g. inside the ::before
and ::after
pseudo-element) using this function.
<blockquote data-mark='"'></blockquote>
In the following CSS block, the character is appended before and after the text inside the element:
blockquote[data-mark]::before,
blockquote[data-mark]::after {
content: attr(data-mark);
}