RawTagHelper.cs

using System.Threading.Tasks;
using Microsoft.AspNet.Razor.TagHelpers;

namespace WebApplication1.TagHelpers
{
    [HtmlTargetElement("raw")]
    public class RawTagHelper : TagHelper
    {
        public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
        {
            var childContent = await output.GetChildContentAsync(false);

            var raw = System.Net.WebUtility.HtmlDecode(childContent.GetContent());
            output.Content.SetHtmlContent(raw);
        }
    }
}

RawTagHelper2.cs

using System;
using System.Text.Encodings.Web;
using System.Threading.Tasks;
using Microsoft.AspNet.Razor.TagHelpers;

namespace WebApplication1.TagHelpers
{
    [HtmlTargetElement("raw")]
    public class RawTagHelper : TagHelper
    {
        private readonly HtmlEncoder _noOpEncoder = new NoOpEncoder();

        public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
        {
            var childContent = await output.GetChildContentAsync(false);

            // Maybe we should just make this work? It doesn't because the TagHelperContent buffer already contains
            // HTML encoded content. This API looks like it should work this way, but doesn't...
            var rawChildContent = childContent.GetContent(_noOpEncoder /* Maybe a null should work here too (no encoding) */);

            // ...so we have to decode it here, which is less than ideal.
            var raw = System.Net.WebUtility.HtmlDecode(childContent.GetContent());

            output.Content.SetHtmlContent(raw);
        }

        private class NoOpEncoder : HtmlEncoder
        {
            public override int MaxOutputCharactersPerInputCharacter
            {
                get
                {
                    return 10;
                }
            }

            public override unsafe int FindFirstCharacterToEncode(char* text, int textLength)
            {
                return -1;
            }

            public override unsafe bool TryEncodeUnicodeScalar(int unicodeScalar, char* buffer, int bufferLength, out int numberOfCharactersWritten)
            {
                numberOfCharactersWritten = 0;
                return false;
            }

            public override bool WillEncode(int unicodeScalar)
            {
                return false;
            }
        }
    }
}