docs: function addClass(element, className) {
    let elements = [].concat(element);
    let classes = [];
    let n = elements.length;
  
    if (!assert((typeof className === 'string') || (className instanceof Array), 'Invalid class name specified. Must be either a string or an array of strings.')) return;
  
    if (typeof className === 'string')
      classes.push(className);
    else
      classes = className;
  
    let nClasses = classes.length;
  
    for (let i = 0; i < n; i++) {
      let e = elements[i];
  
      for (let j = 0; j < nClasses; j++) {
        let c = classes[j];
  
        if (!assert(typeof c === 'string', 'Invalid class detected: ' + c)) continue;
        if (hasClass(e, c)) continue;
  
        e.className = e.className + ((e.className === '') ? '' : ' ') + c;
      }
    }
  }