<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def parse_html(html: str, parser: FragmentParser = None) -> Node: """Parse HTML fragment and return DocumentFragment object. DocumentFragment object has parsed Node objects as its child nodes. """

    parser = parser or FragmentParser()
    parser.feed(html)
    return parser.root