All proposals must first be implemented by some browser vendors at Stage 3:
> The proposal has been recommended for implementation.
Then, the proposal shall be included in the standard at Stage 4:
> Two compatible implementations which pass the Test262 acceptance tests
https://tc39.es/process-document/
reply
So pray tell, what should the browser do? Just sit on their hands, like Firefox? That's a classic example of how a browser could be mismanaged.