Compare Brotli with Zstd

[dennisjackson]

The draft is agnostic as to which compression scheme is used for pass 2 as long as it supports the use of custom shared dictionaries. There's been recent work on a new Brotli format for use with shared dictionaries (IETF draft) which might see use in HTTP transports (WICG draft).

Specific questions:

1. How does the quality and maturity of software implementations compare?
2. How does compression speed / ratio compare?
3. Does Brotli's existing deployment in TLS Certificate Compression give it an advantage?
4. Are existing Brotli implementations forward compatible with the shared dictionary format?