Dark Mode allows users to invert the app setting so that light colored text appears on a dark background instead of a white background. Some users prefer this setting as it can make reading easier on the eyes. Why it matters? Emails are that aren't optimized for dark mode risk looking bad and the first thing a recipient does with a bad looking email is, well [delete].
DKIM allows receiving ESP's to confirm that the sender really is who they say they are and is designed to prevent spoofing. DKIM provides an encryption key and digital signature that confirms the message is coming from an authorized sender.
Google Email Annotations is a great tool for email marketers who really want to leverage their placement in the promotions tab. It allows users to include images, deals, and expiration dates as an extra incentive for email marketers.
Incorporating HTML code to ensure an email looks great across all devices has become a standard for email marketers. This allows email marketers to design one email that looks great everywhere. As mentioned elsewhere, a reader is not going waste time trying to read a bad looking email.