// I tested this PR by error logging the output of wp_register_webfont and 
// wp_register_webfonts in the functions.php file of the currently active theme
$slug = wp_register_webfont(
		array(
			'font-family'  => 'Roboto',
			'font-style'   => 'normal',
			'font-stretch' => 'normal',
			"font-weight" => "900",
			'src'          => array( 'file:./assets/fonts/Roboto-Regular.ttf' ),
		)
	);

// Outputs 'roboto'
error_log( $slug );

$slugs = wp_register_webfonts(array(
		array(
			'font-family'  => 'Roboto',
			'font-style'   => 'normal',
			'font-stretch' => 'normal',
			"font-weight" => "900",
			'src'          => array( 'file:./assets/fonts/Roboto-Regular.ttf' ),
		)
	));

// Outputs [ 'roboto' ]
error_log( $slugs );