WordPress at a glance
filter is not described

category_css_class filter-hook . WP 4.2.0

Filters the list of CSS classes to include with each category in the list.

Usage

add_filter( 'category_css_class', 'filter_function_name_3839', 10, 4 );
function filter_function_name_3839( $css_classes, $category, $depth, $args ){
	// filter...

	return $css_classes;
}
$css_classes(array)
An array of CSS classes to be applied to each list item.
$category(object)
Category data object.
$depth(int)
Depth of page, used for padding.
$args(array)
An array of wp_list_categories() arguments.

Where the hook is called

Walker_Category::start_el()
wp-includes/class-walker-category.php 198
$css_classes = implode( ' ', apply_filters( 'category_css_class', $css_classes, $category, $depth, $args ) );

Where the hook is used (in WP core)

Does not used.