> implementing proposals before they've become standards, and shipping them in their browser.

Is that like commenting before knowing how the TC-39 standardization process works? :P

The proposals are Stage 3. To reach Stage 4 and be complete, there have to be actual, multiple implementations shipping[1].

As the spec page notes, implementations are also almost done in both Firefox[2] and Safari[3].

[1] https://tc39.github.io/process-document/

[2] https://bugzilla.mozilla.org/show_bug.cgi?id=1499448

[3] https://bugs.webkit.org/show_bug.cgi?id=174212