Preview of HTML content not rendered correctly
Prerequisites
- [x] I have searched open and closed issues for duplicates.
Describe the bug
The preview of the email content in the mail list is not displayed correctly if it's a HTML mail. In case of an HTML mail, the HTML code is displayed:

Expected behavior
Displaying just a preview of the text content of the email, without any html tags.
Wanted to tackle this yesterday, but the curent master branch seems to be totally different to what is available on elementary 5.1 Hera. I'm not even able to add an email account.
@ryonakano how to get started playing with the latest Mail codebase?
The latest master branch uses an account that was set up in Switchboard Online Accounts Plug, but the Plug does not support setting up accounts at the moment. You can use Evolution (a mail client by GNOME) to add one as described in README.
I've noticed this as well using a single Office 365 account. I've currently turned off preview.