"These are all specified in DOM model. So in this way, javascript code to visit pages are standardized. We cannot create our own style to visit the elements. In this sense, DOM is like a coding style."

It's not just "a coding style", it is a mandated coding style with a mandated hierarchy. You MUST write the html as the convention specifies so that when Javacript is used on a page, it can interpret the page correctly. It could be call the Document Object Convention.