from pathlib import Path
from bs4 import BeautifulSoup
from markdownify import markdownify

def code_language(pre):
    code = pre.find('code')
    for value in (code or pre).get('class', []):
        if value.startswith('language-'):
            return value.removeprefix('language-')
    return ''

soup = BeautifulSoup(Path('input.html').read_text(), 'html.parser')
article = soup.select_one('main article')
if article is None:
    raise ValueError('No main article element in this input')
Path('output.md').write_text(markdownify(
    str(article), heading_style='ATX', code_language_callback=code_language))