Jen Simmons explains, when using feature queries, outdated browsers will not understand the @supports not clause:

@supports not (display: grid) {
    // code for older browsers
    // DO NOT COPY THIS EXAMPLE
}
@supports (display: grid) {
    // code for newer browsers
    // DID I SAY THIS IS REALLY BAD?
}

If you do, certain outdated browsers will not get the code they need.

Instead, here is the best practice:

// fallback code for older browsers

@supports (display: grid) {
    // code for newer browsers
    // including overrides of the code above, if needed
}