Nearly all clients can view HTML emails, but the number of people who do is a different story.
I know that MS Outlook 2003 junk-email filters don't enable HTML image download in the first place. So if you're using images, there's a drawback. As for statistics, not sure. It's probably the same # of users that use *nix based systems versus windows, or IE vs. Firefox/other browsers.
What advantage would the HTML email give you? Are you using the HTML to "beautify" the document, or is the table in the email a necessity.
If you're not using HTML in the user input form, and thinking that it gets transferred to the email, you're wrong.
I think that Plain text emails get the point across. You can format them as you see fit and use "\t" to input tab spacings, and "\n" for breaks. I haven't used HTML emails, ever, and everyone says they like the layout of my emails. Normally they look like:
***** This is a message from XXX website. *****
***** DO NOT REPLY TO THIS *****
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User Supplied Information:
Full Name: Brett Patterson
Email Address: [email]someone@somewhere.com[/email]
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Subject: [Site_Name] User_Defined_Subject
Comments:
User comments go on from here.
I wrap the text at 78 characters
and indent each line.
---------------------------------------------------
Message sent on date() at time().
~Brett