docs: @Nonnull
    public final HCHead addCSSAt (@Nonnegative final int nIndex, @Nonnull final IHCNode aCSS)
    {
      ValueEnforcer.notNull (aCSS, "CSS");
      if (!HCCSSNodeDetector.isCSSNode (aCSS))
        throw new IllegalArgumentException (aCSS + " is not a valid CSS node!");
      m_aCSS.add (nIndex, aCSS);
      return this;
    }