    def decorator(function):
        """Add ``function`` to the ``PROPERTIES``."""
        if property_name is None:
            name = function.__name__.replace('_', '-')
        else:
            name = property_name
        assert name in KNOWN_PROPERTIES, name
        assert name not in PROPERTIES, name

        function.wants_base_url = wants_base_url
        PROPERTIES[name] = function
        if proprietary:
            PROPRIETARY.add(name)
        if unstable:
            UNSTABLE.add(name)
        return function
Add ``function`` to the ``PROPERTIES``.
weasyprint/css/validation/properties.py