Is a browser obliged to use a DOM to render an HTML page?

I was reading the page about the Document Object Model on Wikipedia.

One sentence caught my interest; it says:

A Web browser is not obliged to use DOM in order to render an HTML document.

You can find the entire context on the page right here.

I don't understand that is there any other alternative to render an HTML document? What exactly does this sentence me开发者_JS百科an?

Strictly speaking IE (at least < IE9) does not use a DOM to render an HTML document. It uses its own internal object model (which is not always a pure tree structure).

The DOM is an API, and IE maps the API methods and properties onto actions on its internal model. Since the DOM assumes a tree structure, the mapping is not always perfect, which accounts for a number of oddities when accessing the document via the DOM in IE.

The primary job of a browser is to display HTML. Most browsers use a DOM; they parse the HTML, create a DOM structure from it (which can also be used in JavaScript) and render the page based on that DOM.

But if a browser chooses not to, it is free to do so. I wouldn't know why, and I certainly don't understand why this line is explicitly mentioned in the Wiki article..