If that's your logic and proprietary compression engines are okay despite not being in any browsers, then 7zip ultra can get it to 69531 bytes? That's 5000+ bytes lower than Zopfli

bzip2 can get it to 65536 bytes which is a whopping 10k smaller than zopfli

Oh wait, I see that zopfli is deflate compatible so that means browsers can still decode it. Fascinating.

I wonder if there's any chance of browsers supported xz? Maybe?

There is a compression-time, decompression-time trade-off whenever you use compression. Xz (or LZMA) is interesting in that its decode time is quite small compared to its encode time, unlike others like bz2. For large, static content the trade-off is quite good. However, with smaller files, like css and javscript sources, you really need to do some benchmarking to be sure.

On-demand xz will probably never be worth it however, the encode time just takes too long. You would have physically transferred the data by the time it has finished compressing.

I was just talking about this with some other Firefox developers today, actually. So...yes, there's a chance. Not a very big one, mind you. But a guy can dream!