def symbols(tokens, base_url):
    """``symbols`` descriptor validation."""
    values = []
    for token in tokens:
        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]))
            continue
        return
    return tuple(values)
``symbols`` descriptor validation.
weasyprint/css/validation/descriptors.py