Don't forget that HTML emails are much larger in size than the plain text equivalents, and available space can be an issue with many people, especially if they want to keep your email rather than delete it.
Plain text can also be quicker to read.
Some recipients prefer a quick message and an invitation to visit your website to a long HTML email that provides the information there and then, whether they want it or not.
Since HTML emails are frequently associated with spamming, some mail services might send the HTML formatted mails straight to the spam folder and you never see them, and other strip the HTML off leaving a text-type format that it extremely difficult to make sense of.
However, HTML does have its uses and many people like to use mainly text with just a small amount of HTML to add stress, underlines and highlight specific parts of the message.
Such simple usage of HTML is also easy to provide in an alternative text form without losing any of the sense of the message.
Do not assume that everybody likes HTML just because you have found it a good way to format your emails.
Many associate it with adverts and spamming, and do not want their emails to look like web pages.
Why not just send them to a formatted web page?That way you do not annoy your recipients and offer them a choice.
In the long run, that may work better for you than an 'in-your-face' brash HTML email.