I've adapted Nikhil's answer somewhat to simplify it. Admittedly, I have not run it through a .net compiler and there are likely very good reasons for the lines Nikhil put in which I have omitted. However, at least in my use case (a very simple page) they were unnecessary.
My use case was for a quick powershell script:
$htmlText = $(New-Object
System.Net.WebClient).DownloadString("<URI HERE>") #Get the HTML document from a webserver
$browser = New-Object System.Windows.Forms.WebBrowser
$browser.DocumentText = $htmlText
$browser.Document.Write($htmlText)
$response = $browser.document
For my case, this returned an HTMLDocument object with HTMLElement objects in it, instead of __ComObject object types (which are a challenge to use in powershell class code) returned by a call to Invoke-WebRequest in PS 5.1.14393.1944
I believe the equivalent C# code is:
public System.Windows.Forms.HtmlDocument GetHtmlDocument(string html)
{
WebBrowser browser = new WebBrowser();
browser.DocumentText = html;
browser.Document.Write(html);
return browser.Document;
}