Use each format for its strengths
HTML can provide structure, links, and visual hierarchy, but some clients block images or display markup differently. Plain text remains readable in minimal clients and helps when formatted content is unavailable.
Do not put important instructions only in an image or styled element. Include meaningful link text, descriptive alternatives for essential images, and a clear text equivalent for actions such as dates, prices, or account steps.
Test readability and consistency
Send a preview to common desktop and mobile clients, checking line breaks, link destinations, and how the plain-text alternative reads on its own. Verify that personalization fields render correctly and do not expose another recipient's information.
Keep legal notices, contact details, and required preference controls consistent across formats. If the platform generates the alternate format automatically, inspect its output rather than assuming it preserves the structure and meaning of the HTML.