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 Format HTML for Gmail. Enhance your emails in different ways. Hyperlinks, numbered lists and quotes can be used. You can show your feelings with emoticons and highlight key passages in ...

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".

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 ...