def additive_symbols(tokens, base_url):
    """``additive-symbols`` descriptor validation."""
    results = []
    for part in split_on_comma(tokens):
        result = pad(remove_whitespace(part), base_url)
        if result is None:
            return
        if results and results[-1][0] <= result[0]:
            return
        results.append(result)
    return tuple(results)
``additive-symbols`` descriptor validation.
weasyprint/css/validation/descriptors.py