CSS has Feature Queries that you can use to experiment new CSS features. If you would like to add custom styles when the browser supports CSS Grid, you write:

@supports (display: grid) {
  div {
    display: grid;
  }
}

Or add custom styles when browser support ::marker:

@supports selector(::marker) {
  li {
    padding-left: .5em;
  }
}

Note that some feature may have better support then @supports selector, that is, ::marker has better support than @supports selector.

I am excited for Feature Query because we can experiment new ideas without breaking legacy browsers.