Construct a DOM Tree from a given HTML page

I'm struggling to get the DOM tree as the output for a given HTML page. For example, if I'm having an HTML page, passing it to a program, I need to construct the HTML DOM tree of that page and need to get the HTML DOM tree same as the browser does as an output.

I went with several articles which talks about DOM, but those explained how to work and do changes with the DOM. For example, use ElementById() to find an html element through an ID. But my intention is different. I need to get the output of HTML DOM tree which is used by the browser before doing the rendering task.