Conflict between Mime Type and Document Type

I have a webform that is causing dramas for IE and Chrome users.

Errors:
1. I'm getting DTD errors in Internet Explorer. See: http://screencast.com/t/VAPuppI75CO
2. And souuce code shows up in Chrome. See: http://screencast.com/t/CqYj2ebdhmR

I don't know why this is happening but it seems to be only happening on the Webform here: http://bit.ly/x3pQ9U

I couldn't find any issues that related to this in the Webform issue queue.

I am worried that it might be happening on other pages that I don't know about (this isn't the only webform on the site).

In the w3c Validator I see...

Conflict between Mime Type and Document Type
The document is being served with the application/rss+xml Mime Type which is not a registered media type for the Document Type. The recommended media type for this document is: application/xhtml+xml

Using a wrong media type for a certain document type may confuse the validator and other user agents with respect to the nature of the document, and you may get some erroneous validation errors. How to fix this problem? One of the following techniques should help:

If you are serving a static file on a Web server, changing its extension should help. Use e.g .xhtml for XHTML or .mml for MathML.
You may need to reconfigure your Web server. This Setting up MIME Types tutorial has instructions for Apache, Tomcat and IIS.
If the document is generated dynamically, the scripting language should allow you to set up the mime type: this article on MIME Types and Content Negotiation has examples in PHP, Perl, and ASP.

Is it possible that Nginx is causing this conflict for me?