(But seriously, Mozilla engineers have warned the Chrome team that they are too rush with the inclusion of Brotli, since that compression wars are heating up. They still proceeded though, which is unsurprising.)
While that might well be one reason, it should be also noted that Zstandard optimizes for the decompression speed with a reasonable compression ratio while Brotli concentrates on the compression ratio at the slight expense of speed (though it is very hard to do a fair comparison). This is evident from their defaults, where zstd uses a fairly low level (3 out of -7..22) and Brotli uses the maximum level (11 out of 1..11). But both have the decompression speeds far exceeding 100 MB/s which is the practical limit for most Internet users, so zstd's higher decompression speed wouldn't matter much in the web context.
Brotli was arguably designed specifically for the web, because it was originally used in the WOFF2 font format and also had a large amount of preset dictionary collected from the web (including HTML, CSS and JS fragments). Zstandard had no such consideration, and while it could be as efficient as Brotli with a correct dictionary it does have a less merit compared to Brotli in the web context.
Brotli has some pretty wild optimizations for web content, including a gigantic (~120 KB) predefined dictionary packed full of sequences commonly used in HTML/JS/CSS content. This gives it a huge advantage on small text files.
It worked for Brotli. As long as the server implements the proper behaviour (only offering it if the browser sends the corresponding Accept-Encoding header) I don't see why this would be a problem.
Sure, barely any server would ever use it, and it could introduce new bugs (or even security vulnerabilities), but there's nothing from a practical point of view that would block browser vendors from adding more compression algorithms.
It's not like Google, Microsoft and Apple are waiting for standardisation to happen. When Google at some point wants Web PCIe, it'll just appear in browsers once Google has finished it, same thing Apple did with WebGPU: as a PoC while the standard is getting finalised.
Mozilla isn't 100% standards compliant either, but it's introducing relatively few bleeding edge features. I think Mozilla is investing very much in areas other browser vendors aren't necessarily interested in, so I don't know any examples for them from the top of my head.
(But seriously, Mozilla engineers have warned the Chrome team that they are too rush with the inclusion of Brotli, since that compression wars are heating up. They still proceeded though, which is unsurprising.)