Brotli is great at compressing static web content, zstd without a dictionary is unlikely to outperform it. For static content you'd probably rather save 5% of space over some decompression costs, since Brotli decompression is fast enough.

Zstd has an advantage if you don't have the CPU to compress at the maximum level, since zstd is generally faster than Brotli at the lower levels.

Even still, for web compression, Brotli has the advantage of already being present in the browsers, so you're betting off using Brotli for web compression as it stands today.