CSS data types - CSS: Cascading Style Sheets
l values (including keywords and units) accepted by css properties and functions. they are a special kind of component value type. in formal syntax, data types are denoted by a keyword placed between the inequality signs "<" and ">". ...

attr() - CSS: Cascading Style Sheets
n 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. if not set, css will use the default value defined for each <type-or-unit>. ...