> The common advice now is to only use CSS that is supported in the last two major versions of each major browser.

Browsers seem to release a major new version like every 2 weeks now. So this statement does no longer hold true. You'll have a lot of users with broken results, if you follow that advise (everyone who uses a ESR or is otherwise limited with updates and behind a month)

I'd say for safety one should target the browsers of the last 4 years at least. You easily can write new CSS —or Javascript— and then use a conversion during deployment to target the lower standards. That's more important for Javascript than for CSS though. If only half of the sites would deploy with `babel` and with some compatibility in mind 80% less of the web would be broken for users with older browsers.