At the moment, a canister cannot serve plain HTML or XML response. If you really want a plain text to be returned from a canister, you can try the proxy solution given by @nometa in another discussion thread. EDIT: just realized that thread was also a reply to your questions 
We are aware that serving plain HTML (or data encoded in other HTTP content type) directly from the canister end point is a popular demand. But the JSON format at the moment contains extra data besides the return value of calling a method, e.g., signature and proof that can help verify the authenticity of the return value.