[filter-effects] url() as pass-through on invalid reference?

Hi,

The 'filter' property can have a list of filter functions including a url() for referencing <filter> elements. What happens if the <filter> element does not exist or is invalid because of other reasons?

Example: filter: blur(4px) url(#invalid) contrast(0.5);

1) Refuse the whole filter chain (as if 'none' was specified)?
2) The url() is just a pass-through and the filter chain continues as if url() was not specified?

Chrome/Safari seem to do the later. I am not sure if that is correct but maybe it is. How does the computed style look like for this property if the url is invalid?

Greetings,
Dirk