Why is that a stupid answer? It points out just one of many problems with relying on HTML email for anything. I don't see you providing anything constructive to the discussion.
From someone that has been forced by my job to work with HTML email formatting and testing I can tell you from experience that no amount of testing will result in consistent HTML display across even the most popular email clients. It's simply not possible. The closest you will ever get is to go back to the table-based layouts and inline CSS and attributes that should have been abandoned in all HTML over a decade ago. If the OP insists on HTML email, plan on testing on dozens of browsers and mail clients on desktop and mobile, and on different OSs as well. Good luck getting any of them to actually look consistent of even usable on small screens unless the messaging is so simple and linear that it could have been conveyed as a plain text email in the first place with a link to a real website for more information.