WordPress at a glance
filter is not described

widget_custom_html_content filter-hook . WP 4.8.1

Filters the content of the Custom HTML widget.

Usage

add_filter( 'widget_custom_html_content', 'filter_function_name_4141', 10, 3 );
function filter_function_name_4141( $content, $instance, $this ){
	// filter...

	return $content;
}
$content(string)
The widget content.
$instance(array)
Array of settings for the current widget.
$this(WP_Widget_Custom_HTML)
Current Custom HTML widget instance.

Where the hook is called

WP_Widget_Custom_HTML::widget()
wp-includes/widgets/class-wp-widget-custom-html.php 152
$content = apply_filters( 'widget_custom_html_content', $content, $instance, $this );

Where the hook is used (in WP core)

Does not used.