Another common issue developers have using HttpHandlers is indicating what type of content is being sent to the browser. This is done by setting the MIME TYPE in the request header. In ASP.NET, whether you are doing it in an HttpHandler or a Page, you can designate the Response.ContentType.
Response.ContentType = "text/html";
You can review a whole list of common Mime types on Wikipedia. I reccomend reviewing this list if you want to get an idea of how content is designated over the web.