code: public Elements getElementsByAttribute(String key) {
          Validate.notEmpty(key);
          key = key.trim();
  
          return Collector.collect(new Evaluator.Attribute(key), this);
      }
docs: Find elements that have a named attribute set. Case insensitive.
  
  @param key name of the attribute, e.g. {@code href}
  @return elements that have this attribute, empty if none