Why is plaintext better than HTML?
HTML emails are mainly used for marketing – that is, emails you probably don’t want to see in the first place. The few advantages they offer for end-users, such as links, inline images, and bold or italic text, aren’t worth the trade-off.
HTML as a vector for phishing
HTML emails allow you to make links which hide the URL behind some user-friendly text. However, this is an extremely common vector for phishing attacks, where a malicious sender makes a misleading link which takes you to a different website than you expect. Often these websites are modeled after the login page of a service you use, and will trick you into entering your account password. In plaintext emails, the URL is always visible, and you can more easily make an informed choice to click it.
Privacy invasion and tracking
Virtually all HTML emails sent by marketers include identifiers in links and inline images which are designed to extract information about you and send it back to the sender. Examine the URLs closely – the strange numbers and letters are unique to you and used to identify you. This information is used to hack your brain, attempting to find advertisements which are more likely to influence your buying habits. HTML emails are good for marketers and bad for you.
Mail client vulnerabilities
HTML is an extremely large and complicated set of specifications designed without emails in mind. It’s designed for browsing the world wide web, on which a huge variety of documents, applications, and more are available. Implementing even a reasonable subset of these standards represents hundreds of thousands of hours of work, or even millions. A large subset (perhaps the majority) of these features are not desirable for emails, and if included can be leveraged to leak information about you, your contacts, your calendar, other emails in your inbox, and so on. However, because of the herculean effort necessary to implement an HTML renderer, no one has built one specialized for emails which is guaranteed to be safe. Instead, general purpose web browsers, with many of their features disabled, are employed in most email clients. This is the number one source of vulnerabilities in email clients which result in information disclosure and even the execution of arbitrary malicious code.
This is a list of 421 remote code execution vulnerabilities in Thunderbird. If you’re bored, try finding one that doesn’t exploit web tech.
HTML emails are less accessible
Browsing the web is a big challenge for users who require a screenreader or other assistive tools to use their computer. The same problems apply to email, only more so – making an accessible HTML email is even more difficult than making an accessible website due to the limitations imposed on HTML emails by most mail clients (which they have no choice but to impose – for the security reasons stated above). Plain text emails are a breeze in comparison for screenreaders to recite, especially for users with specialized email clients designed for this purpose. How do you speak bold text aloud? How about your inline image?
Some clients can’t display HTML emails at all
Some email clients don’t support HTML emails at all. Many email clients are designed to run in text-only environments, like a terminal emulator, where they’re useful to people who spend a lot of time working in these environments. In a text-only interface it’s not possible to render an HTML email, and instead the reader will just see a mess of raw HTML text. A lot of people simply send HTML emails directly to spam for this reason.
Rich text isn’t that great, anyway
Rich text features desirable for end users include things like inline images, bold or italicized text, and so on. However, the tradeoff isn’t worth it. Images can simply be attached to your email, and you can employ things like *asterisks*, /slashes/, _underscores_, or UPPERCASE for emphasis. You can still communicate your point effectively without bringing along all of the bad things HTML emails come with.-ibid