Symbols / repeated elements

BSS right now has linked elements. This is very usefull for elements that appear in many different pages. Where it falls short is when you try to use it for repeating blocks, like grid blocks in a layout. It would be very nice if I could define a block of my html as a "symbol", use many instances of it (for example as a list item in a gallery list), and then: A) change some attributes (like the img href), in each instance separately without breaking the symbol link and B) be able to add a tag (for example a span inside the div) and propagate the addition to all the symbol instances. I know this asks for some complex logic. Maybe when the user makes a change or addition to a symbol instance there could be a dialog that gives the option to make the change/addition global or local.