They implement APIs ahead of other vendors because someone has to do it first and try out an implementation, most standards start out as experimental implementations. But when other major browsers reject it as a standard, the API is usually deprecated.

I'm sure there exist instances of vendors not waiting for agreement, those are rare exceptions because it essentially breaks the standard. Web fragmentation was a big problem for a long time and the major browsers have largely willingly agreed to avoid that. Apple's participation is de facto mandatory, they control a major browser and other major browser vendors won't ship something that Apple has openly declared they will never support.