code: @Nonnull
    @ReturnsMutableCopy
    public ICommonsList <CSSViewportRule> getAllViewportRules ()
    {
      return m_aRules.getAllMapped (r -> r instanceof CSSViewportRule, r -> (CSSViewportRule) r);
    }
docs: Get a list of all top-level rules that are viewport rules (implementing
  {@link CSSViewportRule}).
  
  @return A copy of all contained <code>@viewport</code> rules. Never
  <code>null</code>.