Since Gmail was introduced way back in 2004, many changes have been made to the email provider. One of the first changes was to remove the HTML editor and just keep the WYSIWYG editor. While Gmail doesn’t offer the functionality of an HTML editor, it does still support HTML.
Most email platforms don’t support iframes due to security risks and the static nature of emails. As a result, platforms like Gmail and Yahoo block iframe content. Instead, consider sharing a direct link or using an image of your form linked to its URL in emails.