Starting with AspNetCore 2.0, it's recommended to use ContentResult instead of the Produce attribute in this case. See: https://github.com/aspnet/Mvc/issues/6657#issuecomment-322586885
This doesn't rely on serialization nor on content negotiation.
[HttpGet]
public ContentResult Index() {
return new ContentResult {
ContentType = "text/html",
StatusCode = (int)HttpStatusCode.OK,
Content = "<html><body>Hello World</body></html>"
};
}