But does it mean the 15% is non-standard or just not implemented at all? If it’s not implemented then all of the implementation is 100% standard compliant and if everyone stopped relying on the 15% part then developing for Firefox should make it compatible for rest of the browsers.
Not really, unfortunately. Firefox has only like 85% of the spec implemented, iirc. It is the browser I develop in most, personally, though, fwiw.
But does it mean the 15% is non-standard or just not implemented at all? If it’s not implemented then all of the implementation is 100% standard compliant and if everyone stopped relying on the 15% part then developing for Firefox should make it compatible for rest of the browsers.