original: <a href="hello">world</a><div>wow <div>such depth</div> with another <a href="foo">link</a></div> <h1>and other html stuff</h1>
parsed: <!-- UNIT1 --><a id="a_1" href="hello">world</a><div id="b_2">wow <div id="b_3">such depth</div> with another <!-- UNIT4 --><a id="a_4" href="foo">link</a></div> <h1>and other html stuff</h1>