GitHub Pages currently support on-the-fly gzip compression. Since it’s resource-intensive, usually the compression level is not set to high/max. Users can instead opt for pre-compressing the assets using the maximum compression level. There are Jekyll plugins (1, 2, 3 @philnash) that enable users to do that easily. I’m not sure Pages actually utilize pre-compressed assets or not, but it would benefit Github (in reducing CPU resource and bandwidth) if that’s the case.
All major browsers support brotli since the end of 2017.
TL;DR : Suggest to support gzip and brotli pre-compressed assets and brotli on-the-fly compression.