docs: function ready(callback = null) {
      let promise = new Promise((resolve) => {
          if (document.readyState === "complete") {
              return resolve();
          }
  
          document.addEventListener("DOMContentLoaded", () => {
              resolve();
          });
      });
  
      return (typeof callback === "function") ?
          promise.then(callback) :
          promise;
  }