code: @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;
    }
docs: Add a CSS node at the specified index.
  
  @param nIndex
  The index to add. Should be &ge; 0.
  @param aCSS
  The CSS node to be added. May not be <code>null</code>.
  @return this for chaining