My original advice was blunt: choose plain text instead of a fully designed HTML newsletter. I worried that images would be blocked, messages would land in spam and readers would see a polished email as advertising. These things can happen, but I drew too broad a conclusion about HTML.
Make the message readable on its own
The point of the email should be clear even before images load. Write a recognizable subject, quickly explain why you are writing and use links whose destination is clear. Modest formatting can improve structure and readability. Large banners, many buttons and little real text can distract from the message.
Test the email on a phone and computer. Read it without images: does an offer, explanation or important instruction disappear? Add descriptive text. Check that someone using a simple email client can follow it.
Delivery is more than design
Gmail's current sender guidelines allow HTML messages when formatted to standards. They also stress honest content, clear sender information and understandable links. I would no longer say that HTML alone sends an email to spam. Choose a format that serves the message, then evaluate it with real readers.