Check out Html Tidy for .NET/Mono

From the project page:

TidyManaged

This is a managed .NET/Mono wrapper for the open source, cross-platform Tidy library, a HTML/XHTML/XML markup parser & cleaner originally created by Dave Raggett.

And sample usage:

using System;
using TidyManaged;

public class Test
{
  public static void Main(string[] args)
  {
    using (Document doc = Document.FromString("<hTml><title>test</tootle><body>asd</body>"))
    {
      doc.ShowWarnings = false;
      doc.Quiet = true;
      doc.OutputXhtml = true;
      doc.CleanAndRepair();
      string parsed = doc.Save();
      Console.WriteLine(parsed);
    }
  }
}

Looks like it should meet your needs perfectly.

Answer from Paul Sasik on Stack Overflow
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