Embeder.cs

using System;
using System.Linq;
using System.Net;
using System.Text.RegularExpressions;
using HtmlAgilityPack;

namespace EmbedHtml
{
    public class Embeder
    {
        private const int MAX_CONCURRENCY = 5;

        public string Url { get; set; }

        public Embeder(string url)
        {
            Url = url;
        }

        public string Process()
        {
            var html = new WebClient().DownloadString(Url);
            var doc = new HtmlDocument();
            doc.LoadHtml(html);

            var images = doc.DocumentNode.SelectNodes("//img");
            var imagesParallel = Enumerable.Range(0, images.Count).Select(i => images[i]).AsParallel().WithDegreeOfParallelism(MAX_CONCURRENCY);

            imagesParallel.ForAll(img =>
            {
                var extension = Regex.Match(img.Attributes["src"].Value, "^.(.+)$");
                var imageUri = new Uri(new Uri(Url), img.Attributes["src"].Value);

                var data = new WebClient().DownloadData(imageUri);

                if (data != null && extension.Success)
                {
                    var dataB64 = Convert.ToBase64String(data);
                    img.Attributes["src"].Value = string.Format("data:image/{0};base64,{1}", extension.Value, dataB64);
                }
            });

            return doc.DocumentNode.InnerHtml;
        }
    }
}