2015 is good enough.

For example instead of grid center, one can use flex and margin auto.

If you are building really nation-wide products, there are still a lot of guys in corporate with old windows (where even chrome stopped updating like win7). Or, you know, old or poor people with PC from 2008.

Also don’t forget guys with mobile phones: not like one could easily install a browser there. Especially on phones which no longer receive updates.

So writing CSS like it is 2015 is great. Not because it feels great but because it is what caring about your users (and business) is.

Otherwise you’ll get humbled by your clients soon enough. And in corporate they won’t even be your clients unless you support old stuff: IE 11 is a great target if you really want to shine.

I definitely don't agree with all of these, but grid centering is pretty nice and has a lot fewer quirks than Flexbox based solutions.

I dare you to find any analytics, anywhere, that show any IE 11 usage.

It would be utterly negligent to still be running IE in a corporate environment. It’s a huge security risk.