Source

File: wp-includes/functions.wp-styles.php

function wp_add_inline_style( $handle, $data ) {
	_wp_scripts_maybe_doing_it_wrong( __FUNCTION__, $handle );

	if ( false !== stripos( $data, '</style>' ) ) {
		_doing_it_wrong(
			__FUNCTION__,
			sprintf(
				/* translators: 1: <style>, 2: wp_add_inline_style() */
				__( 'Do not pass %1$s tags to %2$s.' ),
				'<code>&lt;style&gt;</code>',
				'<code>wp_add_inline_style()</code>'
			),
			'3.7.0'
		);
		$data = trim( preg_replace( '#<style[^>]*>(.*)</style>#is', '$1', $data ) );
	}

	return wp_styles()->add_inline_style( $handle, $data );
}