code: public SimpleHTMLTag addSubTag(SimpleHTMLTag tag) {
          if (tag == this) {
              throw new IllegalArgumentException("The tag cannot be its own parent");
          }
  
          if (subTags == null) {
              subTags = new ArrayList<>();
          }
  
          subTags.add(tag);
  
          return this;
      }
docs: Adds a subtag to this tag.
  If the tag has a text content, any subtag will be ignored.
  @param tag Subtag to add
  @return This tag