Sending HTML emails
You can find more information on how to send HTML emails with Symfony mailer on Sending HTML emails with Symfony mailer.
Debugging emails
The Drupal.org documentation handbook has a great article for working with email in a development and testing environment: https://www.drupal.org/docs/develop/local-server-setup/managing-mail-handling-for-development-or-testing
The Drupal Commerce team recommends using Mailhog for local email testing and development. DrupalVM will send emails to it by default and it is easily installed locally or through a Docker container.