I agree that the documentation seems misleading for MvcHtmlString.

However, MvcHtmlString is intended to be used when you don't want a string to be HTML encoded. The default behaviour of razor is to encode the output.

The Html string that you pass to it should already be encoded to ensure that it is outputted without additional encoding.

So assuming the following HTML helper:

public static class HtmlHelper
{     
    public static string GetHtmlString(this System.Web.Mvc.HtmlHelper htmlHelper)
    {
        return "<h1>myHeading</h1>";
    }

    public static MvcHtmlString GetMvcHtmlString(this System.Web.Mvc.HtmlHelper htmlHelper)
    {
        return MvcHtmlString.Create("<h1>myHeading</h1>");
    }
}

With the Razor view:

@Html.GetHtmlString()
@Html.GetMvcHtmlString()

The output would be:

<h1>myHeading</h1&gt

myHeading