Here's another non-obvious gem that seems to have been slipping through the Exchange Team cracks for years now.
On Exchange 2010 if a Resource Mailbox (typically a meeting room) is set to send "Additional Text" in the "Resource Information" settings, the response will always arrive as Plain Text, even if it is configured with proper HTML. However, this will only occur for users who are using Outlook 2010 or 2013 in online mode, rather than the preferred and standard cached mode.
This is a known issue with no current resolution. It does not appear to affect Exchange 20103.
Additionally, due to the way this Resource Information text is wrapped in a special embedded calender response message, the formatting is VERY limited and non-standard; most HTML will not work correctly. It does, whoever, appear to work correctly with most HTML for users who receive the response via Outlook Web App (OWA). Horribly inconsistent and frustrating.