Does anyone know how to send HTML email using Rules? This was a pretty common task in D7 and I think the rule was included with the mimemail module. The D8 version of mimemail doesn't appear to have Rules support.
Mime Mail is not completely ported to D8 yet but there is a patch that ports the Rules support at https://www.drupal.org/project/mimemail/issues/2928965
If you need this feature I suggest you contribute to that issue.