In my Ajax request (using jQuery) I am returning a JSON response.
So json.Html will have a string of HTML I want to append inside a div.
On the server side,
You only need to add slashes to quotes and slashes (ala \" and \\) when escaping content to be put into a JSON string. HTML characters mean nothing inside a JSON string, so they're fine :)
Of course, make sure your string is itself well-formed (X)HTML so that it doesn't explode when inserted into the div.