The library isn't intelligent enough to create the opening p where you put it, but it's intelligent enough to create the missing h1. And in general, it creates valid HTML always, but not always the one you would expect.

So this code:

        HtmlDocument doc = new HtmlDocument();
        doc.Load(yourhtml);
        doc.Save(Console.Out);

will dump this:

<div><h1> hello Hi</h1></div> <div>hi <p></div>

Which is not what you want, but is valid HTML. You can also add a little trick like this:

        HtmlNode.ElementsFlags["p"] = HtmlElementFlag.Closed;
        HtmlDocument doc = new HtmlDocument();
        doc.Load(yourhtml);
        doc.Save(Console.Out);

that will dump this:

<div><h1> hello Hi</h1></div> <div>hi <p></p></div>