How can I return HTML form in BlockBase build function?

Question

I include some form from API. And I want to include it in block, using Drupal\Core\Block\BlockBase. My function return HTML tags, that has tag <form>.

But function build() return empty array.

$element = array(
  '#markup' => $html
);
return $element;

Answer 1

You will have to include your tags in the #allowed_tags, because this markup is filtered in Xss::filterAdmin.

$element = [
  '#markup' => $html,
  '#allowed_tags' => ['form', 'input', ...]
];
return $element;