def string_set(tokens, base_url):
    """Validation for ``string-set``."""
    # Spec asks for strings after custom keywords, but we allow content-lists
    if len(tokens) >= 2:
        var_name = get_custom_ident(tokens[0])
        if var_name is None:
            return
        parsed_tokens = tuple(
            get_content_list_token(token, base_url) for token in tokens[1:])
        if None not in parsed_tokens:
            return (var_name, parsed_tokens)
    elif tokens and get_keyword(tokens[0]) == 'none':
        return 'none', ()
Validation for ``string-set``.
weasyprint/css/validation/properties.py