It seems odd to me that both my Gmail account (in IE and Firefox) and a different email account in Thunderbird display the plain text version over the HTML. I receive lots of HTML email messages in both places. Using the Gmail app on my iPad, it displays the HTML version.
I temporarily removed the customizations I made to the User Posted Email Template to see if maybe something I did there was to blame. However, Gmail still did not display HTML even with the defaults. Neither did the account I use in Thunderbird.