code: @Nullable
    public CSSViewportRule getViewportRuleAtIndex (@Nonnegative final int nIndex)
    {
      return m_aRules.getAtIndexMapped (r -> r instanceof CSSViewportRule, nIndex, r -> (CSSViewportRule) r);
    }
docs: Get the <code>@viewport</code> rule at the specified index.
  
  @param nIndex
  The index to be resolved. Should be &ge; 0 and &lt;
  {@link #getViewportRuleCount()}.
  @return <code>null</code> if an invalid index was specified.
  @since 3.7.4