DOMTokenList.supports()

let trueOrFalse = element.supports(token)

let iframe = document.getElementById('display');

if (iframe.sandbox.supports('an-upcoming-feature')) {
  // support code for mystery future feature
} else {
  // fallback code
}

if (iframe.sandbox.supports('allow-scripts')) {
  // instruct frame to run JavaScript
  //
  // (NOTE: This feature is well-supported; this is just an example!)
  //
}