Remember that there was a time where webpages used badges like these ones as a way to say "sorry, the other browsers suck, download my favorite one or get screwed":

So your assertion is wrong, it is not Safari, but Chrome the browser that is becoming the new IE6:

Believe it or not, Internet Explorer was considered the best browser almost 2 decades ago, Netscape eventually died due to the mindset that IE6 gained, and almost a decade ago many people were using an abandoned and outdated browser, the lack of competition caused stagnation.
This is what is happening with Chrome right now, browsers like Opera ditched their engine in order to use Blink[1] (the engine used by Chrome), even Internet Explorer (now Edge) considered using Blink[2], and Firefox started using it in their UI/UX experiments[3].
Every time we are favoring Chrome the same way we favored IE 6, we are repeating the mistakes from 2 decades ago.
I know it is frustrating when browsers become slow at catching up, but if we neglect the other browsers, we're maligning the future of the web by causing disasters like a mobile web reliant in webkit-prefixed properties[4].
Although to be honest, we're in heaven in comparison with the days where IE 6 was the dominant browser after the death of Netscape.
Right now it is an order of magnitude easier to apply techniques like Progressive Enhancement or at the very least Graceful Degradation[5], so you should relax your requirements, the world won't end if you don't use the latest CSS for your next project.
[1] dev.opera.com/blog/a-first-peek-at-opera-15-for-c…
[2] blogs.windows.com/msedgedev/2015/02/26/a-break-fr…
[3] medium.com/project-tofino/browsers-innovators-dil…