def prefix_suffix(tokens, base_url):
    """``prefix`` and ``suffix`` descriptors validation."""
    if len(tokens) != 1:
        return

    token, = tokens
    if token.type in ('string', 'ident'):
        return ('string', token.value)
    url = get_url(token, base_url)
    if url is not None and url[0] == 'url':
        return ('url', url[1])
``prefix`` and ``suffix`` descriptors validation.
weasyprint/css/validation/descriptors.py