import logging


def stylesheets(soup=None):
    """Find stylesheet URLs in HTML code.

    :param soup: The parsed HTML, defaults to None
    :param soup: BeautifulSoup, optional
    :return: All stylesheet URLs from link tags
    :rtype: list
    """
    if soup is None:
        logging.info('⚠️ No HTML content available.')
        return
    links = soup.findAll('link', attrs={'rel': 'stylesheet'})
    stylesheet_urls = [link["href"] for link in links]
    return stylesheet_urls