WordPress at a glance
function is not described

WP_Customize_Custom_CSS_Setting::validate() public WP 4.7.0

Validate CSS.

Checks for imbalanced braces, brackets, and comments. Notifications are rendered when the customizer state is saved.

{} It's a method of the class: WP_Customize_Custom_CSS_Setting{}

No Hooks.

Return

true|WP_Error. True if the input was validated, otherwise WP_Error.

Usage

$WP_Customize_Custom_CSS_Setting = new WP_Customize_Custom_CSS_Setting();
$WP_Customize_Custom_CSS_Setting->validate( $css );
$css(string) (required)
The input string.

Changelog

Since 4.7.0 Introduced.
Since 4.9.0 Checking for balanced characters has been moved client-side via linting in code editor.

Code of WP_Customize_Custom_CSS_Setting::validate() WP 5.7.2

<?php
public function validate( $css ) {
	$validity = new WP_Error();

	if ( preg_match( '#</?\w+#', $css ) ) {
		$validity->add( 'illegal_markup', __( 'Markup is not allowed in CSS.' ) );
	}

	if ( ! $validity->has_errors() ) {
		$validity = parent::validate( $css );
	}
	return $validity;
}