The blog post documents a relatively long engagement process with the web standards process. Starting with publishing an explainer (pretty common for the browser engines), directly talking with the other browser engines and web developers at the W3C TPAC conference, making concrete proposals to the relevant web standards githubs, taking feedback, etc.

Can you elaborate on the carelessness here? What should've been more careful?

I expect you will apply this to the other browsers who are 100x worse than Apple at adding proprietary features.

And if you read the article Apple reached out to the other browser vendors to push it as a standard.

It's standards track and we've had positive signals from Mozilla and Google. Apple is just the first to ship in this case. Are you also mad when Apple is _not_ the first to ship a feature? Is there any way to win?