HTML emails are tricky even outside Nintex Workflows. Outlook, Gmail, Yahoo, etc all render html differently, so trying to format an email that looks good in all of them is tough.
However, there are a couple of bits that I can suggest that can help.
1. Put content inside a HTML Table: This is probably the smallest amount of work for the biggest gain. If you present your information inside a 700px wide table (transparent) it will render correctly in almost any size view. I've found that 700px is a good size for "mail previews" like Gmail and Outlook. I do this by having a the code (see attached text file), either first in last in your "build string" for email, OR the first thing (and last thing) in any build string repeating process (as well as the ending as noted in file. NOTE: The code also sets a consistent font type and size for anything inside of it.
2. Forget CSS use Inline Styling: Some email clients will respect CSS others won't, easiest way to cover them all is to set any styling directly inline. also, if your primary recipients email client is Outlook, there are a few more quirks, the biggest being that any table header should have "mso-table-lspace:0pt; mso-table-rspace:0pt;" in the inline style tag to make the table render correctly.
3. Treat Email Like Its a Marketing Email: All of the same tips, tricks, and solutions that marketing types use when sending emails will generally solve any issue your having with an html email sent through a workflow. Check Google, you will find a lot of tips and tricks, like this one Coding an Email Template: Tips, Tactics & Trip-ups
I hope this information is helpful,
This document was generated from the following discussion: Formatting Nintex email notifications