As I understand it, html can only be embedded statically. In other words, you’d have to somehow acquire and convert the HTML to something that would compatible to the user. You could not feasibly or easily show Salesforce data in the email in this way.
Here’s a few articles about it.
https://www.lifewire.com/how-to-send-html-email-3468764
https://www.mailup.com/resources-mailup/strategy/strategies-techniques-and-best-practices/creating-a-good-html-email-message/
Have you considered using Conga Composer? It may be better suited for this.
