Implements hook_preprocess_HOOK() for block.html.twig.
function node_preprocess_block(&$variables) {
if ($variables['configuration']['module'] == 'node') {
switch ($variables['elements']['#plugin_id']) {
case 'node_syndicate_block':
$variables['attributes']['role'] = 'complementary';
break;
case 'node_recent_block':
$variables['attributes']['role'] = 'navigation';
break;
}
}
}