Feature detection for proxy browsers that lie about their capabilities

proxy.js

// localStorage example
var hasStorage = (function () {
  try {
    localStorage.setItem(mod, mod);
    localStorage.removeItem(mod);
    return true;
  } catch (exception) {
    return false;
  }
}());
// Streamlined cut-the-mustard method using Page Visibility API (http://caniuse.com/#feat=pagevisibility)
if ('visibilityState' in document) {
  // Modern browser, add JS functionality
}

Additional context about this.