def negative(tokens, base_url):
    """``negative`` descriptor validation."""
    if len(tokens) > 2:
        return

    values = []
    tokens = list(tokens)
    while tokens:
        token = tokens.pop(0)
        if token.type in ('string', 'ident'):
            values.append(('string', token.value))
            continue
        url = get_url(token, base_url)
        if url is not None and url[0] == 'url':
            values.append(('url', url[1]))

    if len(values) == 1:
        values.append(('string', ''))

    if len(values) == 2:
        return values
``negative`` descriptor validation.
weasyprint/css/validation/descriptors.py