Apologies, I'd changed the default font in Google, but this is what the default then becomes on resetting it:
Content-Type: multipart/alternative; boundary=e89a8ff1cd6a68a81104e58fce47
--e89a8ff1cd6a68a81104e58fce47
Content-Type: text/plain; charset=UTF-8
Reset settings, this is now default
--e89a8ff1cd6a68a81104e58fce47
Content-Type: text/html; charset=UTF-8
<div dir="ltr">Reset settings, this is now default<br></div>
--e89a8ff1cd6a68a81104e58fce47--The HTML will force a an HTML enabled client to show the email in whatever default font the user has chosen for their client, so yes in some it may look plain text, but not always if someone changes it.
What I would miss is that when for example, you open it up in Outlook, if you reply you can't then insert images or do anything above a simple reply without finding the menu to change the format to "HTML". Outlook 2013 at least realises what you are doing and asks (but how many use 2013) and I'm pretty sure Thunderbird is the same. The webmail client for MDaemon shows as text, but then on replying converts to "HTML" using the code in the original email.
Anyone would think I was asking for Rich Text Format support
