code: public ContainerTag with(DomContent child) {
          if (this == child) {
              throw new RuntimeException("Cannot append a tag to itself.");
          }
          if (child == null) {
              return this; // in some cases, like when using iff(), we ignore null children
          }
          children.add(child);
          return this;
      }
docs: Appends a DomContent-object to the end of this element
  
  @param child DomContent-object to be appended
  @return itself for easy chaining