Elements can have their own unique properties along with individual validation methods. You can refer to the available elements through the Drupal.org API documentation page at https://api.drupal.org/api/drupal/elements/. However, the classes can also be examined, and the definition method can be read to learn about the properties of each element. These classes are under the \Drupal\Core\Render\Element namespace located in /core/lib/Drupal/Core/Render/Element:
