code: public SimpleHTMLTag setProperty(String key, String value) {
          key = UniformUtils.checkPropertyNameAndLowerCase(key);
  
          if (properties == null) {
              properties = new HashMap<>();
          }
  
          properties.put(key, value);
  
          return this;
      }
docs: Sets a property of the tag by key.
  
  @param key Property key
  @param value Property value
  @return This tag