When modifying Email Designs, the default syntax is Markdown. If you want to use HTML to format, you cannot also use Markdown within the HTML tags. The current workaround is to close out of HTML tags, insert Markdown formatting, then re-open HTML tags. This makes the Email Design unnecessarily complex and messy. However, I haven't found a way to insert AssureSign's default buttons (i.e. to delegate signing) without Markdown, and I want to use HTML to format other aspects of the email, so I am stuck using both.
I recommend that Nintex enhances the Email Design formatting options so that Markdown works within HTML tags.
Another limiter with AssureSign's current Email Designs is that you cannot use style classes. To efficiently write HTML formatting, you can normally define style classes in one spot, and then call the defined style by name later. If you try to write style classes in the Email Design right now, the AssureSign page stops responding and does not recover.
I recommend that Nintex enhances the Email Design formatting options so HTML style classes can be used.