I have a newsletter signup form built over webform. For people who signed up, I want to send an automatic notifications email once the form is submitted. It's working like a plain text. But I would like to customise the email and send in html format. Is it possible to do?
Yes you can! You'll need some more Drupal modules/ config - here is a pretty good tutorial: https://www.ostraining.com/blog/drupal/html-emails-from-drupal-webform/
Note (if this was re: Contribution, Membership or Events): webform_civicrm module will also check to see if you have the Thank you and Receipting section filled out on your CiviCRM side - get into your message templates (HTML version) to make edits to that - if you go that route!