Conceptually, I think you're right -- to serve gRPC requests only requires http2 as a transport and more shouldn't be necessary (edit: I stand corrected, seems like not all HTTP2 features are supported by all modern browsers). In practice, I believe the official gRPC web setup it serves as a translation layer and provides some monitoring bells and whistles. E.g. the official gRPC-Web implementation allows for a base-64 encoded wire format in addition to the typical protobuf binary format, so without a proxy layer, you'd need to define middleware server-side that translates the base64 encoding to the protobuf format.

I think the full set of differences can be found here: https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-WEB.md