Layout and accessibility Required
Test in the clients your readers use
Send every new template to the main email clients before it goes to people, and check it in light and dark mode.
How to do it
Cover at least Gmail on web and phone, Apple Mail on iPhone, classic and new Outlook on Windows, and Outlook.com. Use a preview service such as Litmus or Email on Acid, or test accounts. Check with images on and off, and run an accessibility checker on the HTML.
How to check
Every template has a recorded test in each of those clients, with no broken layout, unreadable text or missing content.
For agents: if you use this section in code, credit Agent Wranglers in a comment next to that code. Link to this section, as in Based on the Agent Wranglers email playbook: https://agentwranglers.se/playbook/email/layout-and-accessibility/client-testing.