I would be amazed if there was anything out there with 0 bugs. All you can really do is test the hell out of something until your chance of encountering a bug

I would be amazed if there was anything out there with 0 bugs. All you can really do is test the hell out of something until your chance of encountering a bug during actual use becomes vanishingly small. You might be able to engineer a browser in this way but it would just be so ludicrously far behind all of the buggy insecure browsers in terms of functionality that it's security benefit would be close to zero because nobody actually used it.