How often have you received emails that showed you this?
Sorry, you need a HTML email client to view this email.
Now, I could save the email to an .eml file, and then use a plain text editor to read the .eml file and decipher the contents. But how many people would care to do that? Mind you, I am using a modern HTML-capable email reader but the incoming email was not properly formatted like most well-formatted HTML emails.
HTML emails are nice to have, and can look pretty on many computers or mobile devices. However, they demand much more bandwidth than plain text emails, and if done incorrectly, renders the email unreadable to some.
A properly formatted HTML email should conform to the baseline HTML conventions, and provide an alternative plain text version of the email. In this fashion, someone with an email client that cannot read the badly formatted HTML email, will still be able to read your email via plain text.
In most direct marketing campaigns, you would notice the availability of receiving emails in plain text or HTML format. However, the option to select such a choice should be AFTER initial interactions in plain text. This is to prevent sales and interactions blocked due to a technical glitch created from your end.
At the end, ask yourself a question - are you looking for sales and interactions from your users, or do you just want to stare at an arguably "pretty" email?