HTML tags not supported when sending emails in Drupal 8
You can't send HTML email with Drupal 8 "out of the box."

To send HTML email, you can use the Swift Mailer module, which has the following features:


  
  Sending e-mails directly through a SMTP server of your choice, a
  locally installed MTA agent such as sendmail or the mail functionality
  provided by PHP.
  Sending HTML (MIME) e-mails.
  Adding file attachments to e-mails.
  Adding inline images to e-mails.
  


The Commerce 2 documentation includes a tutorial on how to install and configure the Swift Mailer module.