LoadHtml(RichEdit1.Text);

procedure TForm1.LoadHtml(html: string);
var
  ms: TMemoryStream;
begin
  ms:=TMemoryStream.Create;
  ms.Write(html[1], Length(html));
  ms.Position:=0;
  try
    WebBrowser1.Navigate('about:blank');
    while WebBrowser1.ReadyState < READYSTATE_INTERACTIVE do
      Application.HandleMessage;
    (WebBrowser1.Document as IPersistStreamInit).Load(TStreamAdapter.Create(ms));
  finally
    ms.Free;
  end;
end;