Could you tell us the exact steps you are using to create the file and upload it? i.e. What applications, webpages, etc.
I didn't do anything exotic. All I did was:
1. sign in to cPanel for my domain, cletus.x10hosting.com,
2. opened File Manager,
3. switched to the sandbox subdomain,
4. created a new file called index.shtml,
5. opened the HTML editor on it,
6. typed in the code I mentioned in the other reply, and
7. saved the file.
A moment later, I used my browser to display the page at
http://sandbox.cletus.x10domain.com/index.shtml and it displayed fine. However, when I did a "view source" on the page, I saw that it had replaced my DOCTYPE and added two new <link> tags and one new <script> tag.
I didn't get any error or warning messages along the way. I can't think of any way that my file could have changed as a result of anything _I_ did. I can only assume that the server somehow overwrote it but I don't understand why. Do you folks have some kind of logic in the HTML Editor that assumes the user is a newbie and that overwrites his work to use a standard doctype that you specify and also adds the link and script tags? Otherwise, I'm not sure how the server would have changed it either!