The parsing time issue is "in the standard now". Even if no rule today matches `property: value { something in brackets }`, that's still at least three tokens (`property`, ':', 'value') to read before bailing out at a bad "property" because values "shouldn't" have things in brackets. (You can build examples with complex CSS selectors where it becomes way more than 3 tokens, as well.)

CSS was designed to be "forgiving" of malformed input, so there's generally no "early" bailout the parser can make, even if it doesn't think it knows which property you are talking about and has a whitelist of specific properties it supports.