If gRPC would have been designed slightly different, they could have had good proxy support AND browser support right from the start.

E.g. it's already based on top of HTTP(/2), and uses normal path for distinguishing methods, which would actually be a good prerequisite to make it work everywhere. But then OTOH it uses barely support HTTP features like trailers, which require very special HTTP libraries and are not universally supported. If the status codes there would have been implemented as just another chunk of the HTTP body, and if some other small changes had been done, we could have had gprc from browsers already a long time ago. I guess that's what grpc-web now tries to fix, but I haven't dug into that in detail.