def marks(tokens):
    """``marks`` property validation."""
    if len(tokens) == 2:
        keywords = tuple(get_keyword(token) for token in tokens)
        if 'crop' in keywords and 'cross' in keywords:
            return keywords
    elif len(tokens) == 1:
        keyword = get_keyword(tokens[0])
        if keyword in ('crop', 'cross'):
            return (keyword,)
        elif keyword == 'none':
            return ()
``marks`` property validation.
weasyprint/css/validation/properties.py