JavaScript and Email. When writing an email the two main choices that you have are to write the email in plain text or to use HTML.
With plain text all you can place in the email itself is text and anything else must be an attachment. With HTML in your email you can format the text, incorporate images and do most of the same things in the email that you can do in a web page. Guide to CSS support in email - Articles & Tips. Version history 14 November 2017 Outlook.com and the Outlook iOS app added support for CSS background images and some related properties, as well as certain Flexbox and Grid properties.
Also added iOS 11 Mail to the guide, with no noticeable differences from iOS 10.