from html.parser import HTMLParser


class MarkedText(HTMLParser):
    def __init__(self):
        super().__init__(convert_charrefs=True)
        self.depth = 0
        self.parts = []

    def handle_starttag(self, tag, attrs):
        if tag == "mark":
            self.depth += 1

    def handle_endtag(self, tag):
        if tag == "mark":
            self.depth -= 1
            assert self.depth >= 0, "Непарный закрывающий mark"

    def handle_data(self, data):
        if self.depth:
            self.parts.append(data)


def extract_marked(source):
    parser = MarkedText()
    parser.feed(source)
    parser.close()
    assert parser.depth == 0, "Незакрытый mark"
    return "".join(parser.parts)


fixture = "<p>Noise <mark>LAB</mark><em>skip</em><mark>{own_example}</mark></p>"
assert extract_marked(fixture) == "LAB{own_example}"
assert extract_marked("<mark>A<strong>B</strong>C</mark>") == "ABC"
assert extract_marked("<p>unmarked</p>") == ""
assert extract_marked("&lt;mark&gt;X&lt;/mark&gt;") == ""
print(extract_marked(fixture))