My point is that treating it as the tree it is, is the only way to really make it impossible to produce invalid HTML. You could also actually validate not just syntax, but also semantic.

> Not a lot of tree oriented output as you make it libraries.

That was actually the point of my library, although I must admit, I haven't implemented actually streaming the HTML output out, before having composed the whole tree. It isn't actually that complicated, what I would need to implement would be to make part of the tree immutable, so that the HTML for it can already be generated.