Litmus user Dan Foody has discovered something very exciting about the Apple Watch. There’s a way to send a hidden version of your email campaign that only the Apple Watch will display. This gives you a lot more flexibility in your design.
Currently, the Apple Watch displays the text version of your emails as the HTML is too complicated for it to process; however, a recent email from Apple included a new content-type that designates watch HTML.
To explain, a typical HTML email contains two parts. One that is text and designated text/plain and another that is HTML, designated text/html. However, Apple's recent email had a third part, designated text/watch-html. This third part is where you can specify Apple Watch formatting for your message.
We took our last newsletter and added a watch-html part. In all other email clients, it looked as if nothing had happened. But, on the Apple Watch, we were able to show a specific, tailored version that utilized the rich text-like support of the Watch.
Unfortunately, the formatting options are limited and images are still somewhat problematic; however, these issues will likely be fixed in the future.
Litmus has a rundown of the supported tags at the link below, including: bold, italics, underline, font color, alignment, quoting, numbered list, bulleted list, and images.