def detect_publisher(fstring):
    """ Detect the publisher of a html file by searching for unique strings
    :param fstring: document HTML or XML text
    :type: byte string
    :returns: publisher name as a unique string
    """
    if b'<meta name="dc.Identifier" scheme="doi" content="10.1021/' in fstring:
        return 'acs'
    elif b'content="10.1039/' in fstring:
        return 'rsc'
    elif b'<link rel="canonical" href="http://www.sciencedirect.com/' in fstring:
        return 'elsevier'
    elif b'full-text-retrieval-response' in fstring:
        return 'elsevier'
    elif b'<meta content="10.1007/' in fstring or b'<meta content="https://link.springer.com' in fstring:
        return 'springer'
    else:
        return None