1 つの div で三角形をつくる
http://www.filamentgroup.com/lab/image_free_css_tooltip_pointers_a_use_for_polygonal_css/
div { width:0; height:0; border-left: 20px solid transparent; border-right: 20px solid transparent; border-top: 30px solid red; border-bottom: 0; }
IE6 だと悲しい結果だけどおもしろい