Revised cut-the-mustard browser feature detection

mustard.js

// Streamlined CTM method using Page Visibility API (http://caniuse.com/#feat=pagevisibility)
if ('visibilityState' in document) {
  // Modern browser, add JS functionality
}

// Previous CTM method based on which DOM features are present
if('querySelector' in document
  && 'localStorage' in window
  && 'addEventListener' in window) {
  // Modern browser, add JS functionality
}