div.mydiv {
  template <Variable mycolor> 
  requires std::is_named_color<mycolor> || is_function_color<mycolor> || is_hexa<mycolor>
  mydiv& background-color: mycolor 
  noexcept( noexcept( background-color: mycolor) || !std::is_IE6 )
  --> decltype (css_declaration<property>)
  ;
}