Feature detection of requestFullscreen() extensions would be possible

[foolip]

The explainer says:

Does not allow developers to detect the Locked Mode feature by checking the existence of a separate API

Although complex, feature detection is possible if we're considering new dictionary members. Example using navigationUI:

let supported = false;
document.createElement('div').requestFullscreen({
  get navigationUI() { supported = true; }
}).catch(() => {});
console.log(supported);

This will make its way through bindings and access navigationUI of the options object passed in, but the request will be rejected and do nothing because the element isn't attached.

[foolip]

It is still absolutely true that detecting a new method is much easier and less of a headscratcher.