使用浏览器提供的localStorage或IndexedDB存储一个随机生成的唯一标识符，每次访问网站时从存储中读取。

是的，这是一种获取浏览器不变的唯一标识符的方法。具体实现可以参考以下代码：

// 生成一个随机的唯一标识符
function generateUUID() {
  var d = new Date().getTime();
  var uuid = 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {
    var r = (d + Math.random() * 16) % 16 | 0;
    d = Math.floor(d / 16);
    return (c == 'x' ? r : (r & 0x3 | 0x8)).toString(16);
  });
  return uuid;
}

// 检查浏览器是否支持localStorage或IndexedDB
function isLocalStorageSupported() {
  try {
    var testKey = 'test';
    var storage = window.localStorage;
    storage.setItem(testKey, '1');
    storage.removeItem(testKey);
    return true;
  } catch (error) {
    return false;
  }
}

function isIndexedDBSupported() {
  return 'indexedDB' in window;
}

// 从localStorage或IndexedDB中读取唯一标识符
function getUniqueID() {
  var uniqueID = '';
  if (isLocalStorageSupported()) {
    var storage = window.localStorage;
    uniqueID = storage.getItem('uniqueID');
    if (!uniqueID) {
      uniqueID = generateUUID();
      storage.setItem('uniqueID', uniqueID);
    }
  } else if (isIndexedDBSupported()) {
    var request = window.indexedDB.open('uniqueID', 1);
    request.onerror = function(event) {
      console.log('Failed to open indexedDB');
    };
    request.onupgradeneeded = function(event) {
      var db = event.target.result;
      var objectStore = db.createObjectStore('uniqueID', { keyPath: 'id' });
      var uniqueID = generateUUID();
      objectStore.add({ id: 1, value: uniqueID });
    };
    request.onsuccess = function(event) {
      var db = event.target.result;
      var transaction = db.transaction(['uniqueID'], 'readwrite');
      var objectStore = transaction.objectStore('uniqueID');
      var getRequest = objectStore.get(1);
      getRequest.onsuccess = function(event) {
        if (getRequest.result) {
          uniqueID = getRequest.result.value;
        } else {
          uniqueID = generateUUID();
          objectStore.add({ id: 1, value: uniqueID });
        }
      };
    };
  } else {
    uniqueID = generateUUID();
  }
  return uniqueID;
}

以上代码中，`generateUUID()`函数用于生成一个随机的唯一标识符，`isLocalStorageSupported()`和`isIndexedDBSupported()`函数用于检查浏览器是否支持localStorage或IndexedDB，`getUniqueID()`函数用于从localStorage或IndexedDB中读取唯一标识符，如果没有则生成一个新的唯一标识符并存储到localStorage或IndexedDB中。