Feature detection?

[superpoincare]

I know old browsers will just ignore something like:

font-display: swap;

in the CSS, but there are cases where feature detection is useful.

For example I want to test the feature and use Bram Stein's fontfaceobserver for browsers which do not support it.

Such as

if(browsersupportsfontdisplay) { //do nothing } else { // run fontfaceobserver. }

So is there a feature detection code?