In order to embed HTML, you actually need to copy the rendered HTML from either an existing web page or WYSIWYG HTML editor and then paste it into Gmail.

How to Make Gmail Display Mail in Basic HTML. Any slowdown in communications can have an adverse effect on a small business owner's bottom line.

Google has quietly announced that it will pull the plug on the basic HTML version of Gmail starting in 2024. We’re writing to let you know that the Gmail Basic HTML view for desktop web and ...

If you prefer viewing your Gmail account in basic HTML, you'll soon be out of luck. Google is removing Gmail's Basic HTML view and replacing it with the Standard view across the board in January 2024.

It's unclear when Google decided to discontinue Basic HTML support, but information about it can be found on this support page titled "Use the latest version of Gmail in your browser".