Progressive enhancement is a wonderful thing. I try to make sites usable in browsers of that era (with a TLS terminator) despite using several HTML 5 and bleeding-edge CSS features. Every feature possible should be progressive.
Here’s the compatibility statement for seirdy.one
I’m not asking anything too radical: when you want to use a feature, just try to make support optional. If there are two ways to do something, have a bias towards the older way. Without trying, you’ll get good support for these browsers and for extensions that modify pages.
As a baseline, I recommend starting with the subset of the HTML Living Standard that appears in the abandoned HTML 5.1 standard. CSS should be optional. This tends to progressively degrade fairly well.