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.