See http://jsbin.com/wirubeq/edit?html,console,output. In Chrome/Firefox, when a component with light DOM content is attached to the document, and a slot is created with a slotchange listener, the slot will raise an initial slotchange event. Safari does not do this. The Chrome/Firefox behavior is useful because it allows a component to concentrate its initial content processing in a slotchange handler. To get consistent behavior, we have to normalize the slotchange behavior across browsers, which is cumbersome.